a fuel injection port of the invention is provided with a seal member enlarging surface which face to a seal member to seal outer circumferential surface of a nozzle and enlarges the seal member . further a moving member which moves in following the nozzle is installed , and the seal member or the seal member enlarging surface is moved by moving member . when the nozzle is inserted , abutting of both members is performed thereby the seal member is enlarged and the nozzle does not interfere with the seal member . after the nozzle is inserted , both members are separated and the seal member is closely contacted with outer circumferential surface of the nozzle .