PURPOSE: To reduce a signal leaked to the outside of a microcomputer and to reduce spurious radiation by providing a logic circuit with a high gain and a logic circuit with a low gain and applying switching control at application of power and after the stable oscillation.

CONSTITUTION: The title circuit consists of a feedback amplifier comprising a crystal oscillator 1, a 1st logic circuit 4 made up of an inverter with a high gain and a 2nd logic circuit 5 made up of an inverter with a low gain, and of a phase detection control circuit 8 applying switching control to the 2nd logic circuit 5 in response to the phase of an oscillated signal 8. Then the oscillator 1, a feedback resistor 2 and a capacitor 3 are mounted externally to a CPU 10 and the logic circuits 4, 5 and the phase detection control circuit 6 are built in the inside of a CPU 10. The logic circuits 4, 5 are used at application of power to oscillate the oscillator 1 and the oscillation of the logic circuit 4 with high gain is stopped by the control circuit 6 after the stabilized oscillation and the oscillated element 1 is oscillated with the logic circuit 5 with a low gain only. As a result, the signal leaked to the outside of the CPU 10 is reduced.