To provide a safety device for a side dump truck, which is capable of surely preventing the deformation and turning-over of a car body by mounting front and rear load boxes on a car body and regulating the dumping of the other load box while one load box is dumped.

A safety device is provided with a front limit switch for detecting the dumping of a front load box, and a rear second limit switch for detecting the dumping of a rear load box. When the dumping of the rear load box is detected by the rear second limit switch, a control valve 21 is held in a first position P1 for communicating a switch valve 19 with a rear cylinder 4A and, when the dumping of the front load box is detected by the front limit switch, the control valve 21 is held in a second position P2 for communicating the switch valve 19 with a front cylinder 3A.