Abstract: The present invention provides a self-propelled trailer dump truck which can convey earth and sand at an amount substantially corresponding to two dump trucks by one conveying operation without overloading, whereby excellent effects can be obtained on an economical field and an environmental field. The fixed platform self-propelled trailer TR is provided with a power engine 9 for self-propelling, a mechanism such as a steering system and the like, a dumping mechanism OS for taking off a load and a load box 6 connected to said dumping mechanism OS, wherein an open driver's seat 8 is positioned in front of said load box 6, a trailing member 12 provided in a front end side of a chassis 7 is provided so as to freely rise and fall, and the trailing member 12 is stood up at a time of self-propelling.