A lens including: a lens body having first and second surfaces; a flange part formed to protrude from a periphery of the lens body and to have a cylindrical outer circumferential surface; a lens installation surface formed at an outer edge part of the flange part; a recessed part formed as a part of the flange part such that a part of the cylindrical outer circumferential surface is recessed toward the optical axis; and a gate root part located at a central part of the recessed part. The gate root part has a flat surface which is in a same level with respect to the lens installation surface. The gate root part lies on an optical axis side with respect to a virtual curved surface formed by extending the cylindrical outer circumferential surface through the recessed part.