To provide a gas burner capable of preventing corrosion of a gas nozzle and a fuel gas supply line by back-flow.

This co-combustion burner comprises a heavy oil burner mounted on a center of a fire hole in a state of penetrating through a wind box and injecting heavy oil toward a furnace, a plurality of gas nozzles mounted in parallel with the heavy oil burner in a state of surrounding the heavy oil burner, and injecting a fuel gas toward the furnace, a cylindrical burner throat mounted in a state of surrounding at least a part of the plurality of gas nozzles, a combustion air introducing portion, and an air damper for adjusting a size of the combustion air introducing portion. The combustion air introducing portion may be an opening formed on a side face of the burner throat or a gap existing between an end portion of the burner throat and an inner face plate or an outer face plate of the wind box.