A high-frequency power supply device generates a high-frequency signal, periodically controls the amplitude or phase of the generated high-frequency signal, and outputs high-frequency power, magnitude of which is controlled on the basis of the high-frequency signal, amplitude or phase of which has been controlled. The high-frequency power supply device controls the amplitude or phase of the high-frequency signal such that the magnitude of the high-frequency power is a first level in a first period of a control cycle and is a second level in a second period of the control cycle which is different from the first period. The second level is lower than the first level. The high-frequency power supply device gradually decreases or increases at least one of the second level and the ratio of the length of the first period to the length of the control cycle, and gradually increases or decreases the first level.