In a refrigeration cycle system, temperature sensors measure the temperatures Tin and Tout of a refrigerant at the entrance and exit of an evaporator. A control circuit performs PID calculation based on signals from the temperature sensors in order to control a degree of superheat corresponding to the difference between the temperatures Tin and Tout at the entrance and exit to an aimed value, and calculates an operation amount for the opening degree of an expansion valve. In a case where the operation amount exceeds the limit of realizable velocity of driving an actuator that actuates the expansion valve, the control circuit performs a limiter process for limiting the operation amount so as not to exceed the limit value. The control circuit drives the actuator by using the operation amount after subjected to the limiter process, thereby to stably control the opening degree of the expansion valve.