A plasma processing apparatus includes a chamber, a window plate disposed in an upper portion of the chamber and having a fastening hole defined therein, an injector having a body part including a plurality of nozzles and configured to be fastened to the fastening hole, and a flange part extending radially from the body part to partially cover a bottom surface of the window plate when the body part is fastened to the fastening hole, and a stopper configured to be fastened to the body part on an upper surface of the window plate to hold the injector in the fastening hole when the body part is fastened to the fastening hole.