A gas laser oscillator includes an output command part for outputting a power supply output command; a power supply unit for applying a discharge tube voltage corresponding to the power supply output command to the discharge tube to start discharge; a voltage detection part for detecting the discharge tube voltage; a discharge start determination part for determining whether or not the discharge has been started on the basis of a value detected by the voltage detection part when the power supply output command is gradually increased; and a sealability determination part for determining that sealability of the gas container has been reduced when a discharge start time from the application of the discharge tube voltage until when it is determined by the discharge start determination part that the discharge is started exceeds a predetermined first threshold value.