A plasma processing apparatus having a sample bench located in a vacuum chamber,
a structure disposed at a position opposed to a sample placed on the sample bench
and facing a plasma generated in the vacuum chamber, and at least one through-hole
disposed in the structure through which a gas flows into the vacuum chamber. An
optical transmitter is mounted on a back of the at least one through-hole through
which light from the sample passes, which light is detected by way of the optical transmitter.
