To prevent the sliding performance of a needle valve sliding part from being deteriorated due to a counter flow of an injected fluid by minimizing a seal oil outlet flow outflowing to an injection nozzle side and detecting the counter flow of the injected fluid in advance to prevent the counter flow, in a high-temperature and high-pressure fluid injection valve applied to an internal combustion engine.

The high-temperature and high-pressure fluid injection valve reciprocates a needle valve to open and close a flow path that is formed in a valve body, and is used for a high-temperature and high-pressure fluid. In a sealing method for preventing the high-temperature and high-pressure fluid from entering a portion between the valve body and the needle valve, there are monitored the pressure P2 of the high-temperature and high-pressure fluid in a feeding channel 47 for feeding the high-temperature and high-pressure fluid f to the valve body 30, the seal oil pressure P1 in a feeding channel 43 for feeding seal oil s to the portion between the valve body 30 and the needle valve 31, and the temperature T in a seal oil feeding channel in the feeding channel 43. A ratio between the pressure P2 of the high-temperature and high-pressure fluid and the seal oil pressure P1 is held within a preset range by regulating the seal oil pressure in the feeding channel for feeding the seal oil or the pressure of the high-temperature and high-pressure fluid inside the feeding channel 47 for feeding the high-temperature and high-pressure fluid. Furthermore, the temperature in the feeding channel 43 for feeding the seal oil is held at or below a preset value.