This paper proposes The Rainbow Marker, a planar marker for estimating the direction of a light source using a structural color. A structural color is a color produced by microscopically structured surfaces that vary in appearance according to the viewpoint, the direction and the spectrum of the light source. The proposed marker contains a planar material which causes structural coloration. The direction of the light source is estimated by structural color pattern matching between an input pattern and referential color patterns. In this paper, two types of the marker were implemented, with a grating sheet and with a holographic sheet, to demonstrate that the proposed method is applicable in the field of augmented reality.