When controlling a clock to a central processing unit, it is naturally preferable not to change existing circuits which are not related to the control of the clock. A clock control circuit inputs a clock input CLK, and generates a clock output CLKOUT for the central processing unit. In the clock control circuit, CLK is masked by a write operation to an internal register. CLKOUT is stopped by the mask operation. The mask operation is carefully designed to be initiated when the internal cycle of the central processing unit is detected. The resumption of the clock is initiated by interrupts.