diff --git a/README.md b/README.md
index 0103dbca..1e876bab 100644
--- a/README.md
+++ b/README.md
@@ -123,7 +123,7 @@ if err != nil {
 
 ## Requirements
 
-This library requires Go 1.7 or later.
+This library requires Go 1.10 or later.
 
 ## LICENSE
 
diff --git a/linebot/flex.go b/linebot/flex.go
index aeed7a48..458a3d6d 100644
--- a/linebot/flex.go
+++ b/linebot/flex.go
@@ -17,7 +17,12 @@ package linebot
 import "encoding/json"
 
 // FlexContainerType type
-type FlexContainerType string
+type FlexContainerType = string // alias type
+
+// IntPtr is a helper function for using *int values
+func IntPtr(v int) *int {
+	return &v
+}
 
 // FlexContainerType constants
 const (
@@ -323,7 +328,7 @@ type BoxComponent struct {
 	Type     FlexComponentType
 	Layout   FlexBoxLayoutType
 	Contents []FlexComponent
-	Flex     int
+	Flex     *int
 	Spacing  FlexComponentSpacingType
 	Margin   FlexComponentMarginType
 }
@@ -334,7 +339,7 @@ func (c *BoxComponent) MarshalJSON() ([]byte, error) {
 		Type     FlexComponentType        `json:"type"`
 		Layout   FlexBoxLayoutType        `json:"layout"`
 		Contents []FlexComponent          `json:"contents"`
-		Flex     int                      `json:"flex,omitempty"`
+		Flex     *int                     `json:"flex,omitempty"`
 		Spacing  FlexComponentSpacingType `json:"spacing,omitempty"`
 		Margin   FlexComponentMarginType  `json:"margin,omitempty"`
 	}{
@@ -351,7 +356,7 @@ func (c *BoxComponent) MarshalJSON() ([]byte, error) {
 type ButtonComponent struct {
 	Type    FlexComponentType
 	Action  TemplateAction
-	Flex    int
+	Flex    *int
 	Margin  FlexComponentMarginType
 	Height  FlexButtonHeightType
 	Style   FlexButtonStyleType
@@ -364,7 +369,7 @@ func (c *ButtonComponent) MarshalJSON() ([]byte, error) {
 	return json.Marshal(&struct {
 		Type    FlexComponentType        `json:"type"`
 		Action  TemplateAction           `json:"action"`
-		Flex    int                      `json:"flex,omitempty"`
+		Flex    *int                     `json:"flex,omitempty"`
 		Margin  FlexComponentMarginType  `json:"margin,omitempty"`
 		Height  FlexButtonHeightType     `json:"height,omitempty"`
 		Style   FlexButtonStyleType      `json:"style,omitempty"`
@@ -426,7 +431,7 @@ func (c *IconComponent) MarshalJSON() ([]byte, error) {
 type ImageComponent struct {
 	Type            FlexComponentType
 	URL             string
-	Flex            int
+	Flex            *int
 	Margin          FlexComponentMarginType
 	Align           FlexComponentAlignType
 	Gravity         FlexComponentGravityType
@@ -442,7 +447,7 @@ func (c *ImageComponent) MarshalJSON() ([]byte, error) {
 	return json.Marshal(&struct {
 		Type            FlexComponentType        `json:"type"`
 		URL             string                   `json:"url"`
-		Flex            int                      `json:"flex,omitempty"`
+		Flex            *int                     `json:"flex,omitempty"`
 		Margin          FlexComponentMarginType  `json:"margin,omitempty"`
 		Align           FlexComponentAlignType   `json:"align,omitempty"`
 		Gravity         FlexComponentGravityType `json:"gravity,omitempty"`
@@ -507,7 +512,7 @@ func (c *SpacerComponent) MarshalJSON() ([]byte, error) {
 type TextComponent struct {
 	Type    FlexComponentType
 	Text    string
-	Flex    int
+	Flex    *int
 	Margin  FlexComponentMarginType
 	Size    FlexTextSizeType
 	Align   FlexComponentAlignType
@@ -523,7 +528,7 @@ func (c *TextComponent) MarshalJSON() ([]byte, error) {
 	return json.Marshal(&struct {
 		Type    FlexComponentType        `json:"type"`
 		Text    string                   `json:"text"`
-		Flex    int                      `json:"flex,omitempty"`
+		Flex    *int                     `json:"flex,omitempty"`
 		Margin  FlexComponentMarginType  `json:"margin,omitempty"`
 		Size    FlexTextSizeType         `json:"size,omitempty"`
 		Align   FlexComponentAlignType   `json:"align,omitempty"`
diff --git a/linebot/flex_test.go b/linebot/flex_test.go
index ebf6199b..8973e401 100644
--- a/linebot/flex_test.go
+++ b/linebot/flex_test.go
@@ -334,7 +334,7 @@ func TestUnmarshalFlexMessageJSON(t *testing.T) {
 								&TextComponent{
 									Type:   FlexComponentTypeText,
 									Text:   "4.0",
-									Flex:   0,
+									Flex:   IntPtr(0),
 									Margin: FlexComponentMarginTypeMd,
 									Size:   FlexTextSizeTypeSm,
 									Color:  "#999999",
@@ -353,14 +353,14 @@ func TestUnmarshalFlexMessageJSON(t *testing.T) {
 										&TextComponent{
 											Type:  FlexComponentTypeText,
 											Text:  "Place",
-											Flex:  1,
+											Flex:  IntPtr(1),
 											Size:  FlexTextSizeTypeSm,
 											Color: "#aaaaaa",
 										},
 										&TextComponent{
 											Type:  FlexComponentTypeText,
 											Text:  "Miraina Tower, 4-1-6 Shinjuku, Tokyo",
-											Flex:  5,
+											Flex:  IntPtr(5),
 											Size:  FlexTextSizeTypeSm,
 											Wrap:  true,
 											Color: "#666666",
@@ -375,14 +375,14 @@ func TestUnmarshalFlexMessageJSON(t *testing.T) {
 										&TextComponent{
 											Type:  FlexComponentTypeText,
 											Text:  "Time",
-											Flex:  1,
+											Flex:  IntPtr(1),
 											Size:  FlexTextSizeTypeSm,
 											Color: "#aaaaaa",
 										},
 										&TextComponent{
 											Type:  FlexComponentTypeText,
 											Text:  "10:00 - 23:00",
-											Flex:  5,
+											Flex:  IntPtr(5),
 											Size:  FlexTextSizeTypeSm,
 											Wrap:  true,
 											Color: "#666666",
@@ -431,6 +431,7 @@ func TestUnmarshalFlexMessageJSON(t *testing.T) {
 						},
 					},
 					Spacing: FlexComponentSpacingTypeSm,
+					Flex:    IntPtr(0),
 				},
 			},
 		},
diff --git a/linebot/send_message_test.go b/linebot/send_message_test.go
index 8a40bc03..3e829f32 100644
--- a/linebot/send_message_test.go
+++ b/linebot/send_message_test.go
@@ -384,6 +384,7 @@ func TestPushMessages(t *testing.T) {
 								&TextComponent{
 									Type: FlexComponentTypeText,
 									Text: "world",
+									Flex: IntPtr(0),
 								},
 								&SpacerComponent{
 									Type: FlexComponentTypeSpacer,
@@ -396,7 +397,7 @@ func TestPushMessages(t *testing.T) {
 			ResponseCode: 200,
 			Response:     []byte(`{}`),
 			Want: want{
-				RequestBody: []byte(`{"to":"U0cc15697597f61dd8b01cea8b027050e","messages":[{"type":"flex","altText":"this is a flex message","contents":{"type":"bubble","body":{"type":"box","layout":"vertical","contents":[{"type":"text","text":"hello"},{"type":"text","text":"world"},{"type":"spacer"}]}}}]}` + "\n"),
+				RequestBody: []byte(`{"to":"U0cc15697597f61dd8b01cea8b027050e","messages":[{"type":"flex","altText":"this is a flex message","contents":{"type":"bubble","body":{"type":"box","layout":"vertical","contents":[{"type":"text","text":"hello"},{"type":"text","text":"world","flex":0},{"type":"spacer"}]}}}]}` + "\n"),
 				Response:    &BasicResponse{},
 			},
 		},