Abstract: Provided is a substrate assembly which satisfies both highly precise positioning and easier assembling. A substrate assembly, formed by positioning a substrate and installing the substrate onto a predetermined member, is configured such that the predetermined member comprises a projection which protrudes toward the substrate, the substrate comprises a hole at a position corresponding to the projection into which the projection is fitted, the projection comprises a small projecting portion on a sidewall surface, and the small projecting portion and a sidewall surface of the projection on the reverse side of the small projecting portion are configured so as to come into contact with a wall surface of the hole to thereby determine a position of the substrate to the predetermined member in a predetermined direction.