A plasma processing apparatus having a sample stage disposed inside a vacuum chamber and a plate member disposed opposing to a sample which is placed on the sample stage and supplied with electric power. The sample is processed using a plasma generated between the sample stage and the plate member and a measuring port is disposed at a back side of the plate member. The measuring port includes an optical transmitter which receives light from a surface of the sample, and a seal which vacuum-seals between an atmospheric side and vacuum side of the vacuum chamber.