In order to prevent particles within a unit from sticking to a substrate in a substrate processing process, an ion generator charges the particles. At the same time, a direct current voltage of the same polarity as the charged polarity of the particles is applied from a direct current power source to the substrate. In order to prevent generation of particles when producing gas plasma, a high-frequency voltage is applied to the upper and lower electrodes at multiple stages to produce plasma. In other words, at a first step, a minimum high-frequency voltage at which plasma can be ignited is applied to the upper and lower electrodes, thereby producing a minimum plasma. Thereafter, the applied voltage is increased in stages to produce predetermined plasma.