A pilot air passage, a pilot fuel passage and a main fuel passage are formed in a gun head for supplying pilot air and fuel. At least one of a flow amount detecting orifice, a flow amount adjusting needle valve and a pressure detecting plug is provided in each of the pilot air passage, the pilot fuel passage and the main fuel passage and is coupled to the gun head so as to be handled together with the gun head. A pilot air tube, a pilot fuel tube and a main fuel tube are constructed in the form of a triplet tube.