A gas laser oscillator, including a power supply part supplying a discharge tube voltage corresponding to a power output command to a discharge tube so as to start a discharge in the discharge tube; a voltage detecting part detecting the discharge tube voltage; a current detecting part detecting an output current of the power supply part; a command voltage control part gradually increasing a power output command value output from the output command part; and a discharge start judgment part judging if the discharge in the discharge tube has started, based on a detected value of the voltage detecting part when gradually increasing the power output command value. The command voltage control part, if the output current becomes a predetermined threshold value or more when gradually increasing the power output command value, decreases the power output command value once, then again gradually increases the power output command value.