To provide a flip-up gate door compact and also capable of easily coping with a heavy-weight gate door.

A flip-up gate door comprises: braces 1; swing arms 3 of which base ends are supported by the braces 1 so that the arms are rotatable around a horizontal rotary shaft 4 in a vertical plane; a door 2 attached to the tips of the swing arms 3; and a door opening auxiliary mechanism 5 accumulating power when the door 2 descends and utilizing the power as a power for opening the door 2. A door opening auxiliary mechanism 5 includes: a chain 6 moving in association with the rotation of the rotary shaft 4; a plurality of energization member fitting parts 7 provided side by side along the chain 6; sprockets 30 detachably provided in at least one position among the plurality of energization member fitting parts 7 and having an output side end 8a engaged with the chain 6; and energization members 8 respectively locked to the sprockets 30.