A parallel circuit 27 of a frequency control circuit 11 is provided as an external circuit. Thereby, the charging time t1 depends on the characteristics of the circuit elements, which are provided in the package of a semiconductor device 70 and therefore subject to manufacturing variations of the semiconductor device 70. The discharging time t2 depends on the parallel circuit 27, which is provided external to the semiconductor device 70 and therefore can be selected to have appropriate characteristics after the semiconductor device 70 has been manufactured. The circuit constants of circuits are set so that the discharging time t2 that depends on the device characteristics of the external parallel circuit 27 is longer than the charging time t1 that depends on the device characteristics of the internal circuits of the semiconductor device 70.