diff --git a/subscribe_message.go b/subscribe_message.go
index c846999..f0499d4 100644
--- a/subscribe_message.go
+++ b/subscribe_message.go
@@ -1,10 +1,14 @@
 package overflow_subscriber
 
-import "encoding/json"
+import (
+	"encoding/json"
+	"fmt"
+)
 
 type SubscribeMessage struct {
-	Targets    []string         `json:"targets"`
-	MessageRaw *json.RawMessage `json:"message"`
+	TargetType SubscribeTargetType `json:"targetType"`
+	Targets    []string            `json:"targets"`
+	MessageRaw *json.RawMessage    `json:"message"`
 	Message    []byte
 }
 
@@ -13,3 +17,53 @@ type SubscribeMessageBody struct {
 	Method  string      `json:"method"`
 	Params  interface{} `json:"params,omitempty"`
 }
+
+type SubscribeTargetType int
+
+const (
+	MEMBER_SESSION SubscribeTargetType = iota
+	MEMBER_ID
+	PROBE_ID
+)
+
+var (
+	subscribeTargetTypeID = map[SubscribeTargetType]string{
+		MEMBER_SESSION: "MEMBER_SESSION",
+		MEMBER_ID:      "MEMBER_ID",
+		PROBE_ID:       "PROBE_ID",
+	}
+
+	subscribeTargetTypeName = map[string]SubscribeTargetType{
+		"MEMBER_SESSION": MEMBER_SESSION,
+		"MEMBER_ID":      MEMBER_ID,
+		"PROBE_ID":       PROBE_ID,
+	}
+)
+
+func (st SubscribeTargetType) String() string {
+	return subscribeTargetTypeID[st]
+}
+
+func (st *SubscribeTargetType) MarshalJSON() ([]byte, error) {
+	value, ok := subscribeTargetTypeID[*st]
+	if !ok {
+		return nil, fmt.Errorf("Invalid EnumType[%s] value", *st)
+	}
+	return json.Marshal(value)
+}
+
+func (st *SubscribeTargetType) UnmarshalJSON(b []byte) error {
+	// unmarshal as string
+	var s string
+	err := json.Unmarshal(b, &s)
+	if err != nil {
+		return err
+	}
+
+	value, ok := subscribeTargetTypeName[s]
+	if !ok {
+		return fmt.Errorf("Invalid EnumType[%s] value", s)
+	}
+	*st = value
+	return nil
+}