To be able to promote oscillation start characteristic while having a simple constitution and maintaining low power consumption. In starting to oscillate an oscillation circuit, a capacitor for protecting from leakage is shortcircuited by bringing a switch of an oscillation stabilizing circuit 11 into a closed state and a constant voltage circuit starts to oscillate the oscillation circuit by supplying a drive voltage of a power source voltage to the oscillation circuit. After an elapse of a predetermined time period, oscillation is prevented from being stopped by leakage by inserting the capacitor into the oscillation circuit loop by bringing the switch into an open state, and an oscillation operation is continued by supplying a voltage lower than the power source voltage to the oscillation circuit by intermittently driving the constant voltage circuit.