A fly-eye lens in accordance with the present invention comprises a plurality of lens elements which are arranged in two-dimensionally close contact with each other so as to connect light-entering and light-exit surfaces of the lens elements to each other, while at least three refractive surfaces with a predetermined refractive power are disposed between the light-entering and light-exit surfaces in each of the lens elements. Here, each of the plurality of lens elements has an identical focal length f and, concering a bundle of rays which enters each of the lens elements in parallel to its optical axis with a maximum incident height h.sub.max and then is emitted therefrom with an exit angle .theta..sub.max, substantially satisfies a conditional expression: h.sub.max =f.multidot.sin .theta..sub.max Accordingly, the plurality of lens elements substantially satisfy a sine condition, whereby the fly-eye lens forms a perfect diffusing surface light source.