To reduce labor of manufacturing by simplifying a structure of a traction force transmission mechanism.

In the opening/closing auxiliary device for the gate plate, a base body 1 fixed to a lower side of a load-carrying platform M of a motor-truck is provided, and an arm mechanism having a first arm 11 front-downwardly rotated at release of the gate plate N is provided on a front side of the base body. A traction force generation mechanism for generating traction force for pulling a traction member to a rear side at advancement of the traction member 23 is provided on a rear side of the base body, and a traction force transmission mechanism for transmitting the traction force of the traction force generation mechanism to the arm mechanism. In the traction force transmission mechanism, a guide roller 31 is mounted to the base body, a curvable traction unit 33 such as a chain, a wire rope and a belt is wound on the guide roller, a rear end of the traction unit is connected to the traction member of the traction force generation mechanism, and a front end of the traction unit is connected to the first arm of the arm mechanism. When the gate plate N is released, a guide roller front side portion of the traction unit is descended while advancing, a guide roller rear side portion of the traction unit is advanced, and the traction force is generated on the traction force generation mechanism.