PURPOSE: To obtain accurate vehicle state input information even when the slip rate of one wheel side increases by correcting the wheel rotating speed on the side where rotary acceleration is large according to the rotating speed on the side where rotary acceleration is small.

CONSTITUTION: Right and left front wheel speed sensors 1 and 2 consist of sensor rotors 11 and 21 which rotate in synchronism with front wheels 4 and 5 and sensor pickups 12 and 22. When the rotors 11 and 21 rotate, permanent magnets in the pickups 12 and 22 generate electromagnetic induced electric power with sine wave voltage signals of frequencies corresponding to the rotating speeds. A module 3 consisting principally of a microcomputer converts a right and left front wheel rotating speed signal which are the sine wave voltage signals from the sensors 1 and 2 into rectangular wave signals to calculate the turning radius, vehicle speed, yaw rate, centripetal acceleration, etc. Then the wheel rotating speed of the side where the rotary acceleration is large is corrected according to the rotating speed on the side where the rotary acceleration is small.