A thin film processing apparatus includes a susceptor and a showerhead facing the susceptor. The showerhead includes a first plate including an inner tunnel, a first injection hole, and a second injection hole. The inner tunnel extends across the first plate in a thickness direction of the first plate. The first injection hole penetrates a first surface and a second surface of the first plate on opposite sides of the first plate in the thickness direction. The second injection hole penetrates the second surface of the first plate. The second injection is connected with the inner tunnel.