An emitting device emits parallel light beams which impinge on a road surface, forming marks. An imaging device takes an image of an image pickup object containing the marks to obtain a picked-up image. A longitudinal distance between mark images respectively corresponding to the marks is detected. A larger gradient can be obtained in the case where the distance is long than in the case where the distance is short. While the gradient of the road surface is obtained based on the distance as described above, a relative positional relationship between the two mark images is not changed as long as the gradient of the road surface is the same even when the height position of an over-spring member provided with the emitting device and the imaging device is changed. Accordingly, the gradient of the road surface can be obtained without consideration of the displacement of the over-spring member.