A circuit breaker including a microcomputer which includes an internal oscillation circuit for generating an internal clock for performing an operation in a low power consumption mode, the circuit breaker comprising further a current transformer arranged such that power coming through the current transformer from current flowing in an AC circuit is supplied to an electric circuit including the microcomputer, wherein the microcomputer is configured to calculate a tripping time corresponding to a level of current flowing in the AC circuit, the circuit breaker further comprising an external oscillation circuit which is configured to generate an external clock with higher speed than that of the internal clock, provided outside of the microcomputer for letting the microcomputer perform an operation in a usual operation mode, a timing circuit which is preset to an oscillation stable time from start-up of the external oscillation circuit up to a time when oscillation of the external oscillation circuit gets stable, and an operation clock selecting circuit which is configured to selectively supply the internal clock from the internal oscillation circuit as an operation clock of the microcomputer at start-up of the microcomputer and the external clock from the external oscillation circuit as the operation clock of the microcomputer after elapse of the time to which the timing circuit is set, wherein the timing circuit is provided outside of the microcomputer and the operation clock selecting circuit is included in the microcomputer, and the timing circuit is formed by a CR delay circuit including a resistor and a condenser, a direct current reference voltage for detecting an output level of an output signal from the CR delay circuit, and a comparator which is configured to compare the output signal from the CR delay circuit with the direct current reference voltage, wherein the timing circuit is configured to be started by applying a clocking start signal (e) from the microcomputer to the resistor at the start-up of the external oscillation circuit, and is configured to output a clocking end signal (f) from the comparator. power coming through the current transformer from current flowing in an AC circuit is supplied to an electric circuit including the microcomputer, wherein the microcomputer is configured to calculate a tripping time corresponding to a level of current flowing in the AC circuit, the circuit breaker further comprising an external oscillation circuit which is configured to generate an external clock with higher speed than that of the internal clock, provided outside of the microcomputer for letting the microcomputer perform an operation in a usual operation mode, a timing circuit which is preset to an oscillation stable time from start-up of the external oscillation circuit up to a time when oscillation of the external oscillation circuit gets stable, and an operation clock selecting circuit which is configured to selectively supply the internal clock from the internal oscillation circuit as an operation clock of the microcomputer at start-up of the microcomputer and the external clock from the external oscillation circuit as the operation clock of the microcomputer after elapse of the time to which the timing circuit is set, wherein the timing circuit is provided outside of the microcomputer and the operation clock selecting circuit is included in the microcomputer, and the timing circuit is formed by a CR delay circuit including a resistor and a condenser, a direct current reference voltage for detecting an output level of an output signal from the CR delay circuit, and a comparator which is configured to compare the output signal from the CR delay circuit with the direct current reference voltage,