Abstract: A plasma processing apparatus is provided with a processing chamber having at least a pair of opposing windows to allow observation of the interior thereof, a plasma generation unit for generating a plasma in the processing chamber, a plasma light emission monitoring unit arranged externally of the processing chamber for monitoring the light emission of the plasma through one of the pair of opposing windows, a reference light irradiation unit for irradiating a reference light to the plasma light emission monitoring unit from that window of the pair of opposing windows which is opposite to the plasma light emission monitoring unit through the pair of opposing windows, and a control for controlling a plasma processing state of the substrate to be processed by comparing the data on the light emission of the plasma monitored by the plasma light emission unit and the reference light.