PURPOSE: To improve sensibility of slip in two wheel drive travelling by sensing the slip with acceleration difference of rotational angle at the sides of front and rear wheels.

CONSTITUTION: When angular speed signal of rount wheel sensor 38 is subtracted from that of rear wheel sensor 39 by a subtracter 41 and the difference is larger than a value set by a comparator 42, or when the angular speed signals are differentiated by differentiating circuits 44, 45 and subtracted by a subtracter 46 and compared with a value set by a comparator 47 to be found larger than the set value, signals for change-over are generated to turn on a switch 42 arranged in parallel to a manual change-over switch 36 for two-four wheel drive. Thus, current is supplied to a solenoid 32 of a valve (not shown) for changing over automatically two-four wheel drive to change over the drive automatically to four wheel drive. When slip is gradual or abrupt, the switch 48 is turned on by the signal of the comparator 42 or 47 respectively.