A plasma processing apparatus includes a chamber; a support member in the chamber; a window plate at an upper portion of the chamber and including a window plate body and a fastening hole, wherein the fastening hole includes a lower fastening hole portion and an upper fastening hole portion. and a gas injector including a first body having a plurality of distribution nozzles and a second body having an accommodating groove to which the first body is fastened and a plurality of injection nozzles. The second body includes a first portion disposed inside the upper fastening hole portion, a second portion disposed inside the lower fastening hole portion, and a third portion disposed below the window plate. The second portion of the second body includes a gas hole extending from the accommodating groove to an external side surface of the second portion of the second body.