To quickly and smoothly supply fuel to a fuel tank in a stable state by supplying fuel to the fuel tank via a relay tank which can be arranged at a location which is lower than the cover part of the tank.

A fuel tank 10 is provided with a relay tank 11 provided with an oil supply port 11a. Fuel can be supplied to the inside of the fuel tank 10 from this relay tank 11 via a suction piping 12, a pumping up pump 13 and a discharge piping 14. When an oil supply nozzle 16 is inserted into the oil supply port 11a, a switch works, the pumping up pump 13 actuates and a state that the oil supplying from the oil supply nozzle 16 is possible is made. When the liquid level of the fuel tank 10 becomes higher than a prescribed value, the value is detected by a level sensor 18 and the actuation of the pumping up pump 13 is stopped.