In a method of suppressing abnormal discharge through a space between a space and a susceptor, a pulse-modulated high frequency wave is supplied from at least one of a first high frequency power supply and a second high frequency power supply. In addition, a DC voltage, which is pulse-modulated in synchronization with the modulated high frequency wave, is applied to the susceptor from a voltage application unit. A voltage value of the modulated DC voltage is set to reduce a difference between a potential of the substrate placed on an electrostatic chuck and a potential of the susceptor.