A processing gas introducing mechanism for introducing a processing gas into a processing space is provided between a plasma generation unit and a chamber of a plasma processing apparatus. The processing gas introducing mechanism includes a gas introducing base having therein a gas introducing path for introducing the processing gas into the processing space, and a near ring-shaped gas introducing plate equipped in the hole part of the gas introducing base such that it can be detached therefrom. Herein, the gas introducing base has a hole part forming one portion of the processing space in a central portion thereof, and the gas introducing plate has plural gas discharge holes communicating with the processing space to discharge thereinto the processing gas from the gas introducing path.