A wafer stage includes a first evaporator where a refrigerant circulates. The first evaporator makes up a cooling cycle with a compressor, first condenser, expansion valve, second evaporator, refrigerant thermometer, and refrigerant flowmeter. The first condenser is supplied with a heat exchange medium. The temperature of a coolant supplied to the second evaporator is measured by an inlet refrigerant thermometer and outlet refrigerant thermometer, while the flow rate of the coolant is monitored and adjusted by a flow-rate adjuster. The temperature difference in the coolant between being at the inlet and at the outlet and flow rate can be measured. Upon complete evaporation of the refrigerant in the second evaporator, the dryness of the refrigerant discharged from the wafer stage is calculated from the amount of heat absorbed from (exchanged with) the coolant, the circulation amount of the refrigerant and the refrigerant temperature to control the rotational speed of the compressor.