To provide a burner and a reactor capable of enhancing combustion efficiency of the burner, and capable of securing stable diffusion combustion by the commonized burner, even when using different combustion speeds of fuels.

This burner 23 of the present invention is provided with a fuel injection port for injecting the fuel into a burner main body, and air injection ports for injecting air on an inner circumferential side and an outer circumferential side of the fuel injection port. In the burner, the different combustion speeds of fuels are injected from the fuel injection port to be diffusion-burnt, in starting and operating, and the burner main body is commonized to cope therewith.