This marker comprises: a plurality of convex surfaces (121) that are formed of a translucent material and that are disposed at least along an X direction; and a plurality of to-be-detected parts which are disposed at front/rear positions with respect to the plurality of convex surfaces (121) and which are projected onto the plurality of convex surfaces (121) as optically detectable images. The plurality of to-be-detected parts are disposed so as to be positioned on the same virtual plane which is perpendicular to a Z direction of the marker. The virtual plane is positioned between a focal point (F) on an image plane (B) of the convex surface (121) in the Z direction and a high point (F) of the image plane.