A plasma processing apparatus processes a film layer to be processed disposed in advance on a surface of a wafer by using a plasma being switched on and off in a processing chamber in predetermined cycles and periods and includes a detection control unit for detecting a processing amount of the film layer on the surface of the wafer. The detection control unit includes a light source unit, a detection unit, and a film thickness/depth calculation unit. This detection control unit detects a plurality of times an amount indicating the intensity of light on a sample surface at predetermined cycles during a period in which the plasma is switched off while the wafer is being processed and detects a processing amount of the film layer on the sample surface by using the detected amount indicating the light intensity.