A lens positioning mechanism includes: a member holding unit that has a function of holding a second surface of a lens member having a first surface, which is a convex optical surface, and the second surface, which is an optical surface facing the first surface, and biasing the held lens member toward a side of the first surface; and an attitude control unit that regulates edge positions of a plurality of portions on the first surface of the lens member biased by the member holding unit to position the lens member in a predetermined attitude.