PURPOSE: To prevent the deflection of the fuel spraying form and the defect on seat and improve the engine performance by installing a heat insulating means on the periphery of the injection hole of a nozzle body and preventing the accumulation of carbon deposit.

CONSTITUTION: A heat-insulated air layer is formed on the periphery of an injection hole 25, and the combustion heat is transmitted into the vicinity of the injection hole 25, having the temperature markedly reduced. In this case, almost all the parts of the combustion heat received by the part close to the injection hole 25 is transmitted to the rear edge side through a nozzle body 26 and an adapter 22, the further transmitted to a cylinder head 23. Therefore, the combustion heat which is received by the part ranging from the injection hole 25 to a seat part 28 is reduced markedly, and the deterioration of the seat part 28 due to heat and the accumulation of carbon deposit in the range from the injection hole 25 to the seat part 28 can be prevented.