The present invention is directed to a traction control system for a four-wheel drive vehicle, which includes a braking operation detector for detecting operation of a brake pedal, an accelerating operation detector for detecting operation of an accelerator pedal, and a slip detector for detecting a slip of each wheel of the vehicle. A braking force controller is provided for controlling a pressure control apparatus to perform a traction control by supplying the hydraulic braking pressure discharged from an auxiliary pressure source to wheel brake cylinders operatively mounted on the wheels under a slip condition, when the accelerating operation detector detects the operation of the accelerator pedal and the slip detector detects the slip of at least one of the wheels. The braking force controller is adapted to control the pressure control apparatus to communicate all of the wheel brake cylinders with a hydraulic pressure generator such as a master cylinder, when the braking operation detector detects the operation of the brake pedal on condition that the traction control is being performed, with the communication between the pressure generator and all of the wheel brake cylinders blocked. Accordingly, when the brake pedal is depressed during the traction control, or immediately thereafter, the braking operation is effectively initiated.