To accurately detect vehicle behavior from a wheel speed signal even if a vehicle speed changes by providing a means to arithmetically operate the vehicle behavior from the wheel speed signal detected by a wheel speed sensor and a means to change the vehicle behavior operation content according to a vehicle speed detected by a vehicle speed detecting means.

A pitch speed and a roll speed of a vehicle being vehicle behavior is found by a vehicle behavior operation means (b) from a wheel speed signal detected by wheel speed sensor (a). A change is applied by an operation content changing means (d) to change the vehicle behavior operation content to find the pitch speed and the roll speed according to a vehicle speed detected by a vehicle speed detecting means (c) at that time. Therefore, the pitch speed and the roll speed of the vehicle can be accurately detected even if the vehicle speed changes by correcting a detecting error caused when a pulse frequency and a noise component of the wheel speed sensor (a) changes according to the vehicle speed.