The interior of the first reservoir is open to the atmospheric air. A first liquid retaining portion and a first air retaining portion are provided in the first reservoir. The interior of the second reservoir is sealed. A second liquid retaining portion and a second air retaining portion are provided in the second reservoir. A communicating portion allows communication between the second air retaining portion and the first liquid retaining portion. The opening of the communicating portion in the first liquid retaining portion is located lower than a minimum water level of the first liquid retaining portion. The opening of the communicating portion in the second air retaining portion is located higher than a maximum water level of the first liquid retaining portion.