In a four-wheel drive traveling system for a tipper, an output side of an engine is connected to rear wheels via a power transmission, at least one variable hydraulic motor for driving the front wheels and a hydraulic pump driven by the engine are provided, and the variable hydraulic pump and the variable hydraulic motor are connected into a closed circuit. The system includes arrangements for controlling displacement of the variable hydraulic pump, for detecting slippage of the rear wheels, for setting a discharge pressure of the hydraulic pump between a high pressure and a low pressure, and for controlling the displacement of the variable hydraulic pump so that the discharge pressure is a low pressure while slippage is not caused for the rear wheels and so that the discharge pressure is a high pressure while slippage is caused for the rear wheels.