A cover portion (96) for covering an opening of a tank body serving as a water tank is provided with a water receiving section (97) formed of a recess for temporary reception of supplied water, and the water receiving section (97) is provided with mesh-like filters (93a, 93b). Thus, scale or other impurities contained in discharge water derived from a pot of a steam generator and discharged from a drain valve are filtrated by the filters (93a, 93b), by which the discharge water from the pot is recycled as water for steam generation without using any special purification system. Therefore, the number of times of water supply to the water tank can be reduced without using any special purification system.