The invention is intended to provide a downhill speed control system which employs a method of setting a target vehicle speed in advance, thereby improving controllability when a vehicle runs down a slope, and giving an operator improved operability in setting of the target vehicle speed. Brakes are controlled so that, when the vehicle runs down the slope, an actual vehicle speed is matched with a target speed set by a switch which can set the target speed by selectively or continuously changing plural preset speeds. Also, control constants for PID control are modified depending on settings of a downslope gradient setting switch and a load setting switch. When an acceleration computed from the actual vehicle speed is larger than a target acceleration, the strength of applied brake is increased.