The object of the present invention is to provide a plasma processing apparatus wherein plasma is generated in process chamber to treat a sample. Said plasma processing apparatus is further characterized in that multiple closely packed through-holes are formed on the plate installed on the UHF antenna arranged opposite to the sample, an optical transmitter is installed almost in contact with the back of the through-holes, and an optical transmission means is arranged on the other end of said optical transmitter, thereby measuring optical information coming from the sample and plasma through optical transmitter and optical transmission means by means of a measuring instrument. No abnormal discharge or particle contamination occur to through-holes even in long-term discharge process, and no deterioration occurs to the optical performance at the end face of the optical transmitter. Said plasma processing apparatus ensures stable and high precision measurement of the state of the surface of sample and plasma for a long time.