A method for adjusting a detection axis of an object detection system, wherein a disk-shaped adjusting member is fixed to a reference plane on an outer face of a casing, wherein the rotational position of the adjusting member is adjusted. The adjusting member has first and second base surfaces, between which a wedge shape is formed. Rotating the adjusting member finely adjusts the angle of the second base surface relative to the reference plane of the casing, thereby resulting in the direction of the second base surface coinciding with the direction of an object detection axis of a radar mechanism part fixed within the casing. The direction of the object detection axis can be adjusted in the horizontal direction by placing a level on the second base surface of the adjusting member and adjusting the mounting angle of the casing so the level indicates a horizontal direction.