A controller of a plasma processing apparatus stores a frequency spectrum related to a first timing into a storage unit, controls a microwave generator to generate a microwave in correspondence to a setting frequency, setting power, and a setting bandwidth at a second timing, controls a demodulator to measure travelling wave power and reflected wave power of the microwave for each frequency, calculates the frequency spectrum related to the second timing on the basis of a measurement result from the demodulator, calculates a correction value for correcting a waveform of the travelling wave power for each frequency such that a difference for each frequency between the frequency spectrum related to the second timing and the frequency spectrum related to the first timing, stored in the storage unit, is small, and controls the microwave generator on the basis of the calculated correction value for each frequency.