To provide an optical low-pass filter which is excellent in MTF characteristics and less in wavelength dependency.

In one unit structure, two unit blocks A of perfect convex structure, two unit blocks B in which thick columns are formed at predetermined pitches, two unit blocks C in which columns of middle thickness are formed at predetermined pitches, two unit blocks D in which slender columns are formed at predetermined pitches and one unit block E of perfect concave structure gather to form one unit structure. A diffusing element is formed by arranging required number of the unit structure in all directions. Thickness of the column and arrangement pitch are shorter than a wavelength of used light. Respective unit blocks of this structure act as blocks having different effective refractive indexes when light is made to pass in a vertical direction of drawing 1. Namely this diffusing board acts as an element having different effective refractive indexes at different places.