To solve a problem that it is difficult to enhance combustion performance when the combustion quantity is throttled because gas and air are not mixed sufficiently when lean mixture gas being supplied to a lean burner port at the top of the burner port member of a lean/rich combustion burner air is produced by diluting rich mixture gas.

In the way of a passage for introducing lean mixture gas to burner ports 41-43 at the top of a burner port member 3, a mixing passage 37 is provided in the vicinity of underside at the lower end of the burner port member 3. A plurality of protrusions 53 are formed to project into the mixing passage 37 at the lower end part of the burner port member 3 while inclining against the vertical direction.