A plasma processing apparatus comprises a processing vessel defined by an outer wall and provided with a stage that holds a substrate to be processed thereon, an evacuation system coupled to the processing vessel, a microwave window provided on the processing vessel as a part of the outer wall so as to face the substrate to be processed on the stage, a plasma gas supplying part supplying a plasma gas to the processing vessel, and a microwave antenna provided on the processing vessel in correspondence to the microwave. The plasma gas supplying part includes a porous medium and the plasma gas supplying part supplies the plasma gas through the porous medium.