PURPOSE: To reduce the current consumption of every oscillation circuit itself by providing the oscillation circuits of plural systems and properly select these oscillation circuits according to the system operating conditions.

CONSTITUTION: Two oscillation circuits 11 and 12 are used for the low and high speed operations respectively. A clock control circuit 16 performs the ON/OFF control of both circuits 11 and 12 according to the system operating conditions. Therefore, the clock signal (CLK1) oscillated by the circuit 11 is supplied to a CPU 14 and a CPU peripheral circuit 15 respectively via a selector 13 as a system clock signal (SYSCLK) in a low speed operation mode. On the other hand, the circuit 12 is kept in a halt state by the oscillation control signal (CLK2BEN0) of the circuit 16.