Fuel injection nozzle and method for manufacturing the same

When the body (3) of the injector nozzle (1) is manufactured, the body (3) is tempered at a predetermined temperature which is higher than a receiving-heat-temperature. A temperature of the body (3) is increased up to the receiving-heat-temperature, receiving a heat from a combustion chamber of an internal combustion engine during the engine is running. The body (3) is hardly softened due to tempering during its operation. When the seat (17) seats on the seat face (16), the seat (17) tends to be abrasive wore.