A plasma processing apparatus includes a temperature measuring unit; airtightly sealed temperature measuring windows provided in a mounting table, for optically communicating to transmit a measurement beam through a top surface and a bottom surface of the mounting table; and one or more connection members for connecting the mounting table and a base plate, which is provided in a space between the mounting table and the base plate. In the plasma processing apparatus, a space above the mounting table is set to be maintained under a vacuum atmosphere, and a space between the mounting table and the base plate is set to be maintained under a normal pressure atmosphere, and each collimator is fixed to the base plate at a position corresponding to each temperature measuring window, thereby measuring a temperature of the substrate via the temperature measuring windows by the temperature measuring unit.