PURPOSE: To improve the responsive quality of a slip control, by providing a bypass passage at a space between the upper stream side of a supercharger at an air intake passage and the upper stream side of a throttle valve, and providing at its halfway point an air feed bypass valve at which a negative pressure source is changed over according to the decision result of a slip presence.

CONSTITUTION: The compressor portion 8 of a turbo charger 7 is provided at the halfway portion of an air intake passage 4, and an intercooler 9 and a throttle valve 10 are provided on its lower stream side. Also, a bypass passage 11 is provided at a space between the upper stream side of the compressor 8 at the air intake passage 4 and the upper stream side of the throttle valve 10, and a valve seat 12 at which the valve body 13 of an air feed bypass valve 14 can be seated, is formed at an opening portion opening to its lower stream side end portion. A negative pressure chamber 16 divided by means of the diaphragm 18 of the air feed bypass valve 14 is made to be connectable to an air intake manifold 2 or a boost tank 21 through a changeover valve 20, and the changeover valve 20 conduct 5 control so that the negative pressure chamber 16 may be changed over to a side connecting to the boost tank 21 by means of a control circuit 23, in the case of a slip presence decision having been made.