The sealed gas laser oscillator that includes the airtight vessel, the laser gas supply source that supplies a laser gas to the airtight vessel, and the vacuum pump that performs evacuation until a pressure of an inside of the airtight vessel reaches a target reached pressure every predetermined interval between laser gas exchanges and is connected to the airtight vessel, and that performs laser oscillation in a state where the airtight vessel is filled with the laser gas. The gas laser oscillator includes a unit that determines the target reached pressure on the basis of the interval between laser gas exchanges, a leakage rate of an impurity gas from the outside to the airtight vessel after evacuation, and an allowable impurity gas pressure at which the gas laser oscillator is capable of being operated.