This marker comprises a plurality of convex-surface section disposed along the X-axis direction and a plurality of detection sections disposed so as to face the convex-surface sections. Each of the detection sections is located, in the protruding direction of the convex-surface sections, closer to the convex-surface section side than a focal point on an image surface of the convex-surface section of the first optical unit in the X direction and is located on a single plane located further on the opposite side from the protruding direction than the edge of the image surface. The image surface extends from the first optical unit to the nth optical unit in the X direction, where n is at least 2.