A gas laser oscillator according to the present invention comprises a resonator unit, a heat exchanger through which a fluid exchanging heat with a laser gas flows, a chiller for cooling the fluid in the heat exchanger and supplying the fluid to the heat exchanger, and a heat transfer device for transferring heat of the fluid to the resonator unit. The gas laser oscillator further comprises a first flow path for supplying the fluid used for cooling the laser gas in the heat exchanger to the heat transfer device, a second flow path for supplying the fluid cooled by the chiller to the heat transfer device prior to supplying the fluid to the heat exchanger, and a switching valve for switching either one of a first flow path and a second flow path.