A fuel tank opening-closing device has a tank opening forming member, a flap valve mechanism that has an opening-closing member, and an opening-closing activation mechanism that switches between the locked position and the unlocked position that does the opening operation of the opening-closing member. The opening-closing activation mechanism has a nozzle detection mechanism which has introductory push parts mounted on the opening-closing member and adapted to receive force of motion in the insertion direction of the fueling nozzle, and a lock member that is mounted on the tank opening forming member and that selectively shifts between the locked position and the unlocked position by engaging and disengaging with the opening-closing member, and a locking mechanism that moves the locking member from the locked position to the unlocked position by being pushed by the introductory push parts.