speed sensors 6a and 6b when this wheel speed difference exceeds a predetermined value the traction control unit 7 judges a slip condition present produces an engine driving torque reduction request signal and delivers this reduction request signal to the three way solenoid valve 21 by this engine driving torque reduction request signal the three way solenoid valve 21 switches from the off state to the on state and makes the atmospheric pressure port p2 closed and the negative pressure port p1 open therefore the negative pressure is introduced into the negative pressure working chamber 18 of the diaphragm unit 15 and causes the diaphragm 16 to move rightward in fig2 against the resilient force of the spring 19 with this rightward displacement of the diaphragm 16 the second throttle valve 4 is brought to the closed position of an approximately 1 8 opening degree the opening degree of the second throttle valve 4 in the closed position is set to a predetermined small degree which is required to repress the slip but which is not so small as to cause an engine stall the setting to about 1 8 is adequate to repress the slip without causing an engine stall a closing time of the second throttle valve 4 is set to a very short time equal to or smaller than 0 2 sec.