A plasma processing apparatus includes a vacuum vessel with a sample stage having a mounting surface disposed in a process chamber, and a plate having substantially uniform thickness and electric power applied thereto constituting a ceiling of the chamber. The plate is disposed opposite to and substantially parallel with the sample stage so as to cover the whole area of the stage mounting surface and has a through-hole therein. An optical transmitter with a diameter larger than a diameter of the though-hole is disposed inside of the vacuum vessel and has an end face at a position above and spaced a small distance a back surface of the plate so as to receive light from the chamber via the through-hole. The optical transmitter is independently detachable with respect to the back surface of the plate.