To provide a buoyant gate for preventing backflow, which can be installed even in a channel of a four-side watertight structure, hardly causes a wire to come off a winding machine drum due to the loosening of the wire, and can surely prevent an unexpected backflow.

The gate 10 for preventing backflow includes: a door body 13 which is arranged on the bottom of the channel in such a manner as to be raised/brought down via a support portion 12 and which produces buoyancy in water; a main wire 14 which has a leading end side connected to the door body 13; and the winding machine 16 which has a main drum 15 for winding/unwinding the base end side of the main wire 14. The backflow prevention gate 10 further includes an urging means 17 which winds the main wire 14 along with the rising motion of the door body 13, and which imparts, to the main drum 15, a winding force for keeping the door body 13 after rising in a rising attitude. The urging means 17 includes an auxiliary drum 25 which interlocks with the rotating shaft 21 of the main drum 15, an auxiliary wire 24 which is wound around the auxiliary drum 25, and a weight 26 which is annexed to the auxiliary wire 24.