A plasma processing apparatus includes an ICP antenna, provided outside a processing chamber opposite to a mounting table, for supplying a high frequency power supply into the processing chamber, and a window member made of a conductor, disposed between the mounting table and the ICP antenna, forming a part of a wall of the processing chamber. The window member includes transmission units for transmitting the high frequency power in a thickness direction of the window member. Each of transmission units has a slit, which extends through the window member in the thickness direction and is configured such that its width is changeable.