A conventional gas laser oscillation apparatus cannot appropriately deal with an amount of laser gas introduced greater than normal use and can generate a turbulent flow around the total reflective mirror and the partial reflective mirror. This causes unstable discharge, resulting in unstable output of laser beams. The gas laser oscillation apparatus of the present disclosure has a laser oscillator, a first laser-gas inlet, a laser-gas outlet, a first laser-gas introducing port, a laser-gas circulation passage, and a second laser-gas introducing port. Disposed between a first discharge tube and any one of the total reflective mirror and a partial reflective mirror, the first laser-gas introducing port introduces laser gas into the laser oscillator. Disposed in the laser-gas circulation passage, the second laser-gas introducing port introduces laser gas into the laser-gas circulation passage. A laser-gas introducing section is connected to the first and the second laser-gas introducing ports.