PURPOSE: To detect a speed in the optimum state, by arranging three or more sensors in a moving direction so as to provide intervals and changing the intervals between the sensors or a sampling cycle to calculate a correlation value.

CONSTITUTION: Sensors 1a, 1b...1n are arranged in an advance direction of a car at a predetermined interval and the sensors of a predetermined interval are selected by a change-over means 5 to calculate a correlation value by an operation processing means 7 to calculate a speed. At this time, correlation values are calculated with respect to all of the combinations of the sensors at first to calculate speeds and a speed is calculated by the combination seeming to be most certain of the sensors. On and after, the sensors are changed over by the means 5 corresponding to the change of a speed so as to become the optimum combination in relation to the number (n) of shifts of data and resolving powder to successively change the interval of the sensors to be used. By this method, the optimum sensor interval is selected corresponding to a speed to calculate a correlation value. A four-wheel drive apparatus 9 is controlled on the basis of the calculated speed.