An unsprung state detection part (140) calculates a variation amount X (=||), which is a magnitude of a difference between a detected angle output from a resolver rotational angle sensor (40) for detecting a rotational angle of an in-wheel motor (30) and an estimated angle of a motor rotational angle (S11 to S13). The estimated angle can be calculated by adding an estimated angle of the rotation of the motor (30) in one calculation cycle to a detected angle n1 of the previous calculation cycle. When the variation amount X is more than a road surface determination threshold Xref, the unsprung state detection part (140) determines that the travel road on which a vehicle (1) is traveling is a rough road (S14, S15). As a result, the road surface determination can be made by using the rotational angle sensor (40).

An inertial measurement sensor system of a vehicle is calibrated during a driving operation of the vehicle and is based on a determination of a misalignment of a sensor coordinate system of the inertial measurement sensor system with respect to a vehicle coordinate system. The determination of the misalignment is interrupted in situations in which a level deviation from a reference level exceeding a predetermined threshold value is determined by means of at least one of the vehicle's own level sensors.