Provided is an ozone generator (47) including two sheets of ozone generating electrodes (156, 157). These two ozone generating electrodes (156, 157) are arranged in a treating passage (152) in series along an air flowing direction. Thus, the air to come in from an air inlet port flows at first along the ozone generating electrode (156) of the first sheet, and receives, while flowing, the creeping discharge of the first ozone generating electrode (156) thereby to generate the ozone. The air thus having generated the ozone further flows in the treating passage (152) to the ozone generating electrode (157) of the second sheet, and receives, while flowing, the creeping discharge of the second ozone generating electrode (157) thereby to generate the additional ozone. The highly dense ozone can be used to clean the washing water.