PURPOSE: To automatically change over a two- or four-wheel drive mode for an appropriate running condition, by detecting whether the road is dry and flat or slippery due to freezed, and the presence or the absence of a tight corner braking phenomenon resulting from steering.

CONSTITUTION: Revolution sensors 12a, 12b, 13a and 13b are provided for left and right front wheel axles 5a, 5b and left and right rear wheel axles 11a, 11b, respectively. A steering angle sensor 16 is also provided for a steering box 15 which is actuated by operating a steering wheel 14. Based on the revolution speeds which are detected by those sensors 12a, 12b, 13a and 13b, a substential slip ratio is calculated. A road condition is estimated on the criteria of the slip ratio and if the value is high, the vehicle is changed over into a four-wheel drive mode. By estimating a slip condition on the basis of a steering angle detected by the steering angle sensor 16, an unintentional change-over into the four-wheel drive mode may be prevented when the vehicle is turned.