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; arms of which base ends are supported by the braces 1 so that the arms are rotatable around a horizontal rotary shaft 2 in a vertical plane; a door attached to the tips of the arms; and a door opening auxiliary mechanism accumulating power when the door descends and utilizing the power as a power for opening the door. In the flip-up gate door, the door opening auxiliary mechanism 3 includes: a driving sprocket 4 fixed to the rotary shaft 2 and a driven sprocket 5 arranged in the lower parts of the braces 1; power transmission means 8 formed in a long annular ring shape having a two linear parts including chains 6, 7 having lengths engaging at least within rotation ranges of the sprockets 4, 5 and two semicircular parts in both ends thereof; and a pair of left and right energization means 9, 10, each having one end fixed to the brace 1 side and the other end fixed to the linear part of the power transmission means 8, and linked to the power transmission means 8 so that the power when the door descends is accumulated while the power can be utilized as the power for opening the door by linking the energization means with the power transmission means 8.