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 air supplied from boundary zone between rich and lean premixed nozzles, produces lower NO_x and CO emissions, although the characteristics of the new burner are clarified only in experiment. 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.