PURPOSE: To continue an operation of a laser by a remaining normal high frequency power source by detecting the malfunction of the high frequency power source according to the output state detected at each power source, and turning OFF the malfunctioning power source according to a malfunction signal.

CONSTITUTION: The output current of a high frequency power source 21 is detected by a current detector 6, this detected current value is amplified by a current amplifier 7, and fed to a calculator 8 and malfunction detecting means 1b. In the means 1b, when the malfunction of the power source 21 is detected by a current signal, a malfunction signal is output to switching means 1a, the output of the power source 21 is turned OFF, and the malfunctioned power source is disconnected from a laser tube. Since the malfunction detecting means are provided at the respective control circuits of one high frequency power source in this manner, even if a malfunction occurs in one power source, it is disconnected to continue the laser oscillation by the remaining power sources.