The method of forming a minute focusing lens with respect to over a photoactive area of an image sensor such as a CCD or CMOS, comprising: coating a resist film on a flattening layer formed over the photoactive area of the image sensor; exposing the resist film to light via a photo-mask, and developing the resist film; and patterning the resist film into a lens configuration provides in this invention in order to form a lens having a designed configuration that provides a good light focusing efficiency. The photo-mask is a light transmission type having no light-shading layer. And, this photo-mask is the one having provided thereon a light transmission portion comprising a light refraction material, having on its surface portion a stairs portion, the stairs portion having the phase of a transmission light at its respective position controlled relative to a prescribed width so that a desired light intensity distribution may be obtained at the surface of the photo-mask light-exposed portion.