To provide a burner capable of deriving a huge amount of combustion heat by properly burning a mixed gas composed of hydrogen and oxygen mixed at a mole ratio of 2:1.

The burner 11 has a burner main body 12 shaped like a rectangular box, a gas supply means provided with a gas supply pipe 14 for supplying a mixed gas 13, and a cooling means for cooling the front end (burner main body 12) of the pipe 14. The gas 13 is supplied to the burner 11 from a gas supplying unit through the pipe 14, and the gas 13 is burned while being discharged from a gas nozzle 27 at the front end of the pipe 14. A coolant 20 which is water or air is let through a cooling channel 36 constituting the cooling means for protecting the burner main body 12 from excessive heating. The nozzle 27 is replaceable, and the cooling means is provided with a flow control valve for the coolant 20.