To provide an opening/closing device for a fuel tank which is capable of smoothly and largely opening a shutter by efficiently using the insertion force of a fuel filler nozzle, miniaturized, and superior in fuel filling work efficiency.

The opening/closing device for the fuel tank has a shutter mechanism 20 which is arranged in a tank opening forming member 11 to open/close a shutter body 51 by inserting the fuel filler nozzle FZ. The shutter mechanism 20 comprises a pressing mechanism 30 having a pressing member 31 arranged in an insertion passage 20P, a connection mechanism 40 having a spring 48 for accumulating the moving force of the pressing member 31 as mechanical energy, and a nozzle detection mechanism 60. The nozzle detection mechanism 60 comprises an introduction-pressing unit 63 for receiving the moving force in the inserting direction by the fuel filler nozzle FZ, and a locking unit 66 for permitting the movement of a shutter 50 to the opening position.

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