To provide a manufacturing method of a mass-producible lens array allowing cost reduction with fewer production steps, to provide a lens array, to provide an illumination optical device, and to provide a projector.

Molten glass G is set and press-molded in a molding die 60 having dies 621A1, 621A2 corresponding to a lens array, so as to form a molded article in which a plurality of lens arrays are integrally molded. After the molding, the molded article is removed from the molding die 60, and a boundary part of the adjacent lens arrays is bent, so as to be separated. Upon the press molding, with at least two side faces among the side faces other than the bend-separation face 121C to be bent and separated in the lens arrays as external position reference faces, the press molding is performed.