An unsprung state detection part (140) calculates a variation amount X (=|φ−φ′|), which is a magnitude of a difference between a detected angle φ output from a resolver rotational angle sensor (40) for detecting a rotational angle of an in-wheel motor (30) and an estimated angle φ′ of a motor rotational angle (S11 to S13). The estimated angle φ′ can be calculated by adding an estimated angle of the rotation of the motor (30) in one calculation cycle to a detected angle φn−1 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).