To quicken the response of the equipment and improve the reliability right after the power source is turned ON or right after a low-speed side 2nd oscillation means begins to operate by making a microcomputer detect whether or not the clock frequency of the 2nd oscillation means is within a specific range by using the clock of a 1st oscillation means.

When the power source is turned ON, high-speed and low-speed side oscillation circuits 4 and 5 begin to operate and when a high-speed side oscillator 2 starts oscillating, a reset circuit 8 is driven to release a main system from being reset, so that a CPU 1 starts operating. A low-speed side clock as the output of the low-speed side oscillation circuit 5 is measured on the basis of a high-speed side clock as the output of the high-speed side oscillation circuit 4 by using a timer in the CPU 1 and, when a state wherein the output of the low-speed side oscillation circuit 5 is within a specific frequency range is detected, it is judged that a low-speed side oscillator 3 oscillates stably, so that the output of the low-speed side oscillation circuit 5 is used for subsequent control.