A slip angle monitoring system directly compares an absolute value of lateral acceleration with an experimentarily derived given value which serves as a slip criterion to determine slip angle of a vehicle. Slip angle is set at zero while the absolute value of the lateral acceleration is smaller than or equal to the given value. On the other hand, when the absolute value is greater than the given value, the slip angle is derived by dividing the lateral acceleration by the longitudinal acceleration or by taking the minus one power of the tangent of the quotient obtained by dividing the lateral acceleration by the longitudinal acceleration, that is, the cotangent of the quotient.