A gas laser oscillating unit having a gas junction part where gas flow may be stable, whereby a stable laser beam output and/or a laser beam that does not fluctuate very much may be achieved. The laser gas, flowing through first and second excitation parts, is introduced into the first and second tapered gas flow passages. After that, the two gas flows are mixed at or near the center point of a gas junction part and the mixed gas flows in a next flow passage. Then, one of the gas flows from the first excitation part is biased toward the −X direction by a first biasing member arranged in the first gas flow passage, and the other gas flow from the second excitation part is biased toward the +X direction by a second biasing member arranged in the second gas flow passage.