A fuel tank opening-closing device is equipped with a flap valve mechanism 20 and an opening-closing activation mechanism. The opening-closing activation mechanism comprises a nozzle detection mechanism that has a first detection parts and a second detection parts. The first detection parts are linked with the locking member and the locked part so as to move the lock engaging part and the locked engagement part while maintaining each of the overlap widths within the preset range, being pressed by the end of the fueling nozzle. The second detection parts are linked to the locking member so as to move from the locked position to the unlocked position by releasing the overlap width by being pressed by the end of the fueling nozzle.