To provide a gas mixer which allows the efficient mixing of air and fuel gas in a flow passage.

A mixing facilitating ring 8 is disposed in the vicinity of a boundary layer 33 generated between an air layer 31 and a gas layer. Karman vortex 34 is generated on the slip stream side after the ring which is placed in the stream and large turbulence occurs. As a result, the boundary layer 33 is largely disturbed and the mixing of the air 21 of both sides and fuel gas 22 is facilitated.