A plasma processing apparatus includes a microwave output unit, a wave guide tube, a tuner, a demodulation unit, and a calculation unit. The microwave output unit outputs a microwave having power corresponding to setting power while frequency-modulating the microwave in a setting frequency range. The wave guide tube guides the microwave to an antenna of a chamber main body. The tuner is provided in the wave guide tube and adjusts a position of a movable plate. The demodulation unit is provided in the wave guide tube, and acquires travelling wave power and reflected wave power for each frequency. The calculation unit calculates a frequency at which a reflection coefficient, which is calculated on the basis of the travelling wave power and the reflected wave power, for each frequency becomes a minimum point as an absorption frequency.