To provide a microlens comprising an array of unit lenses each having a rectangular feature in a plan view formed by a photolithographic step of applying a photosensitive resist on a substrate, exposing the photosensitive resist for patterning by use of a distributed density mask and developing, and to provide a microlens having improved condensing property of the rectangular lenses and efficiently irradiating a photodetector.

The distributed density mask has a slit pattern formed between lens patterns for forming unit lenses adjacent to each other. In each unit lens, the curvature of sphere close to the ends in a diagonal direction of the lens is equal to that of the center portion.