A gas laser oscillator capable of initiating discharge, without applying excess voltage to a discharge tube, and correctly and rapidly judging the initiation of discharge. The oscillator has a laser power commanding part adapted to generate a laser power command including a pulse superimposed on a forefront of each step, a voltage applying part adapted to apply a voltage to a discharge tube based on the laser power command, a discharge tube voltage detecting part adapted to detect the discharge tube voltage, a discharge tube voltage monitoring part adapted to monitor the discharge tube voltage, and a discharge initiation judging part adapted to judge that the discharge is initiated when the difference, between a change rate of the monitored voltage and a change rate of the discharge tube voltage predetermined based on data obtained while the discharge is normally carried out, is smaller than a predetermined threshold.