PURPOSE: To improve the value of a transfer function MTF in the low spatial frequency range of a subject image by forming projection parts on a transparent substrate in stripes so that their sectional shapes are raised smoothly, and specifying the period length, thickness of projections, etc.

CONSTITUTION: The projection width (a) is so specified that 0.5p≤a≤p, where (p) is the period length; and the period length (p) and projection thickness (d) are set while MTF(rc)≤0.3, where MTF(rc) is the value of the transfer function MTF at a cutoff space frequency rc. The projection parts with the width (a) and thickness (d) are formed on the transparent substrate while raised smoothly. For example, a phase type optical low-pass filter 1 has periodic structures formed on both surfaces of the substrate 2 in a cylindrical lens shape. The section of one period of the periodic structures consists of the arcuate projection part 3 and a plane part 4. The direction of stripes consisting of projection parts 3 and plane parts 4, e.g. (y) direction is only at right angles to a direction which a subject image becomes out of focus. When the subject image becomes out-of-focus only in one direction, the periodic structure is formed only one surface of the substrate 2.