A plasma processing apparatus includes a processing container that defines a processing space, a gas supply unit provided on a sidewall of the processing container and configured to supply gas to the processing space, a dielectric member having a facing surface that faces the processing space, and an antenna provided on a surface opposite to the facing surface of the dielectric member and configured to radiate microwaves that turn the gas into plasma to the processing space through the dielectric member. The gas supply unit includes a transport hole transporting the gas to a position where the gas does not reach the processing space in the inside of the sidewall of the processing container and an injection hole communicated to the transport hole and configured to inject the gas transported to the position into the processing space. The injection hole has a diameter larger than that of the transport hole.