An opening degree sensor produces a sensor output corresponding to a valve opening degree of a valve, and includes a memory into which the sensor output corresponding to the valve opening degree is written. When performing a first half process, flow rates of more than one valve device are measured, a flow rate error is calculated based on an average flow rate obtained by averaging the measured flow rates, and the sensor output corresponding to the valve opening degree into which the flow rate error is incorporated is obtained. When performing a second half process, the sensor output corresponding to the valve opening degree obtained in the first half process is written into the memory of the valve device whose flow rate is not measured.