A gas laser oscillator includes a first control valve for controlling an amount of laser gas supplied into a gas container, a second control valve for controlling an amount of laser gas exhausted from the gas container, and a controller for controlling openings of the first and second control valves. The controller includes a storage unit for storing data indicating a relationship between the laser gas pressure in the gas container, the opening of the second control valve, and the exhaust amount of laser gas, a gas pressure control unit for controlling the openings of the first and second control valves, respectively, such that the laser gas pressure becomes closer to a reference gas pressure, and a gas consumption amount control unit for controlling the openings of the first and second control valves, respectively, such that the exhaust amount of laser gas becomes closer to a target consumption amount.