A plasma processing apparatus 31 includes a processing chamber 32; a gas supply unit 33 for supplying a plasma processing gas into a processing chamber 32; a mounting table 34 configured to hold the target substrate W thereon; a plasma generating device 39 configured to generate plasma within the processing chamber 32; and a gas supply device 61. The gas supply device 61 includes a head unit 62 configured to move between a first position above the mounting table 34 and a second position different from the first position and to supply a gas, and the head unit 62 is configured to supply a film forming gas to a small-volume region formed between the mounting table 34 and the head unit 62 when the head unit 62 is positioned at the first position and to adsorb the film forming gas on the target substrate W.