diff --git a/matrix.go b/matrix.go
index 99c7944..e39c9d6 100644
--- a/matrix.go
+++ b/matrix.go
@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"log"
 	"strings"
+	"sync"
 	"time"
 
 	"maunium.net/go/mautrix"
@@ -31,12 +32,17 @@ type MatrixCommand struct {
 }
 
 // Output queue.
-var MatrixOut []MatrixMessage
+var MatrixOut struct {
+	queue []MatrixMessage
+	mux   sync.Mutex
+}
 
 // sendMessage sends a message through Matrix.
 func sendMessage(msg MatrixMessage) {
 	log.Print(msg.text)
-	MatrixOut = append(MatrixOut, msg)
+	MatrixOut.mux.Lock()
+	MatrixOut.queue = append(MatrixOut.queue, msg)
+	MatrixOut.mux.Unlock()
 }
 
 // sendError sends an error through Matrix.
@@ -116,15 +122,16 @@ func initMatrix() {
 
 	go func() {
 		for range time.Tick(time.Second * 2) {
-			if len(MatrixOut) > 0 {
+			MatrixOut.mux.Lock()
+			if len(MatrixOut.queue) > 0 {
 				text := ""
-				for i, msg := range MatrixOut {
+				for i, msg := range MatrixOut.queue {
 					text += msg.text
-					if i != len(MatrixOut) {
+					if i != len(MatrixOut.queue) {
 						text += "\n"
 					}
 				}
-				MatrixOut = []MatrixMessage{}
+				MatrixOut.queue = []MatrixMessage{}
 				_, err := client.SendMessageEvent(context.Background(),
 					id.RoomID(configuration.matrix.room_id),
 					event.EventMessage,
@@ -135,6 +142,7 @@ func initMatrix() {
 					continue
 				}
 			}
+			MatrixOut.mux.Unlock()
 		}
 	}()