When a power supply is turned on, an NPN transistor is disconnected, a first transmission gate is conducted, a second transmission gate is disconnected, and a power-supply voltage is applied to a control input terminal of a voltage controlled oscillation circuit via the first transmission gate. After oscillation of a quartz resonator is stabilized, the NPN transistor is switched to the conducted state by a control signal applied to a general purpose terminal so that the first transmission gate is switched to the disconnected state, and the second transmission gate is switched to the conducted state. Then, a voltage of the voltage control terminal is applied to the control input terminal of the voltage controlled oscillation circuit via the second transmission gate.