Сamera rotates around Z axis, but I don't want that.

Here is my C# script for camera rotation:

     public void CameraRotate(Camera InputCamera, float InputRotationSpeed)
     {
         if (Input.GetButton("Rotate Camera") == true)
         {
             float MouseXMovement;
             float MouseYMovement;
             MouseXMovement = Input.GetAxis("UnityDefaultMouseX");
             MouseYMovement = Input.GetAxis("UnityDefaultMouseY");
             InputCamera.transform.Rotate(-MouseYMovement * InputRotationSpeed, MouseXMovement * InputRotationSpeed, 0);
         }
     }

But if I move cursor diagonally, or make circles with it, the camera also turns around Z axis, despite the fact that Z-rotation is set to zero. How I can avoid that?

Sorry for bad english. I hope you'll understand me.