A gas laser oscillator (2) that excites a laser gas to generate laser light includes a circulation path (9) for the laser gas, a circulation means (14) for circulating the laser gas through the circulation path, a pressure detection means (16) for detecting the pressure of the laser gas in the circulation path, an electric power detection means (11) for detecting electric power to drive the circulation means, a storage means (35) for storing the relationship between the pressure of the laser gas and the electric power of the circulation means during the period of normal operation of the circulation means for each kind of the laser gas, and a laser gas determination means (31). The laser gas determination means determines the kind of the laser gas based on the pressure of the laser gas and the electric power of the circulation means detected during the period of normal operation of the circulation means, and the relationship between the pressure of the laser gas and the electric power of the circulation means stored in the storage means. Due to this, the laser gas is determined without generating a discharge voltage. If the laser gas cannot be determined, it may be possible to judge that the laser oscillator is anomalous.