The apparatus comprises a measured signal detector 11, a vehicle parameter obtainer 12, a sideslip angle temporary estimator 13, a sideslip angle differential corresponding value computer 14 and a sideslip angle real estimator 15. A sideslip angle temporary estimate value is computed from one or plural vehicle parameters including at least the mass. A sideslip angle differential corresponding value is computed from a measured signal and the vehicle parameters including no mass. A sideslip angle is derived from the sideslip angle temporary estimate value and the sideslip angle differential corresponding value. A sideslip can be detected without a steering angle detection mechanism.