A rich-lean flame burner is widely used for supplying domestic hot water and heating in Japan. This burner has low NO_x and CO emissions, and conventionally consists of a layered structure of lean flame with both sides of rich flame. A new concept burner with controlled secondary air supply produces lower NO_x and CO emissions. In this study, a two-dimensional numerical simulation of the new burner is executed to clarify the mechanism of the lower NO_x and CO emissions compared to conventional burner.