Abstract: A tail gate opening/closing device for a dump truck in which bracket means are affixed at the upper rear ends of both side walls in the rear part of the carrier section. One end part of a bracket arm is pivotally arranged at each of the ends of the bracket means and each of the other ends is pivotally fastened to the end of a tail gate arm. When material transported such as sand, soil, gravel, etc. is dumped, the carrier section is raised to an angle with the ground causing the material transported to exert pressure on the tail gate so as to open the tail gate in a rearward direction and, at the same time both the bracket arm and the tail gate arm are rotated upwards and the fulcrum of the tail gate arm is also moved upwards. In this way, the tail gate is opened smoothly and the discharge port is opened to the widest extent possible.