To reduce current consumption by measuring a frequency of a ring oscillator clock with high accuracy, then by operating a microcomputer by only the ring oscillator clock.

An Xin clock count timer 2 starts to count an Xin clock by a start command from a CPU 5. When a counted number reaches a set value, the Xin clock count timer 2 outputs an overflow signal. A ring oscillator clock count timer 4 counts a ring oscillator clock from the generation of the start command in the CPU 5 to the generation of the overflow signal in the Xin clock count timer 2. A frequency of the ring oscillator clock is gotten from a count number counted in the interval.