A disc-shaped filter unit including a filter main body having a water absorbability and a holder having a non water absorbability is disposed in a vertical orientation, with a part of a circumferential portion thereof immersed in water reserved in a water tank, and is rotated by a rotation drive mechanism in a circumferential direction. The filter unit has an absorptive region and a non-absorptive region arranged adjacent to each other in the circumferential direction. Therefore, in accordance with rotation of the filter unit in the circumferential direction, a state where the absorptive region is immersed and the filter main body of the filter unit absorbs water through a water-conduction hole and a state where the non-absorptive region is immersed and water absorption of the filter main body is prevented by a watertight section having a non water absorbability are continuously alternated.