A control unit generates a control signal for controlling an actuator to manipulate a valve element and to control a quantity of flow in a passage according to both an opening position of the valve element and a predetermined correlation between the opening position and the quantity of flow. The control unit includes a storage unit storing an error-characteristic function and a compensation unit to compensate the control signal according to the error-characteristic function. An actual correlation between the opening position and the quantity of flow is actually measured separately from the predetermined correlation. The error-characteristic function is an approximation of a relation between an opening position x in the predetermined correlation and an opening position y in the actual correlation at a same quantity of flow, which are calculated beforehand with reference to the actual correlation and the predetermined correlation.