A microlens array of high converging efficiency is provided, independently of the array and lens filling rate of microlens arrays, with a method of manufacturing microlens arrays using the diffusion process. A multitude of refractive-index distribution type microlenses formed by diffusing in a planar transparent substrate a substance contributing to increasing the refractive index of the substrate are two-dimensionally and regularly arranged on the surface of the substrate. The microlenses are densely arranged on the surface of the substrate, and diffusion fronts of the microlenses form regions where the diffusion fronts are fused with those of the adjoining microlenses. The length of a region where certain two adjoining microlenses are fused together, in the direction of a straight line connecting the centers of the two microlenses is less than 20% of the array pitch of the microlenses in the above-mentioned direction.