A plasma processing apparatus and a temperature measuring method that may measure a temperature of an object in a processing chamber by a low-coherence interferometer without forming a hole in a holding stage or an upper electrode of the plasma processing apparatus, thereby performing a plasma process of a substrate with high precision and uniformity. The plasma processing apparatus is implemented by disposing a light source collimator outside of a light source window, disposing a light-receiving collimator outside of a light-receiving window, allowing a measurement light emitted from the light source collimator to pass through the light source window to be obliquely emitted to a surface of the object to be measured, and allowing the reflected measurement light to pass through the light-receiving window to be incident on the light-receiving collimator. The temperature of the object in the processing chamber may be measured by the low-coherence interferometer.