PURPOSE: To provide a resin lens capable of suppressing fluctuation in a refractive index distribution in a direction perpendicular to an optical axis even if ambient humidity changes.

CONSTITUTION: An fθ lens made of a resin has a long-sized shape relatively large in the width of the first direction (Z-axis direction) perpendicular to the optical axis O. The lens face 1F on a light incident side is a plane and the lens face 1R on an exit side is a convex spherical face. The side faces exclusive of the lens faces, i.e., the long side faces 1H, 1H parallel with the XY plane along the X-axis direction and the short side faces 1V, 1V parallel with the XY plane are all planes. Moistureproof layers 10, 10 are formed on the long side faces 1H, 1H.