Disclosed is a manufacturing method of a complex lens for a tandem scanning optical system that includes a step for preparing molding dies for forming a cavity to form the complex lens as a single-piece element, and a step for injecting lens material into the cavity. The molding dies include a pair of single-piece mirror surface cores that form a plurality of lens surfaces of the complex lens at an incident side and a plurality of lens surfaces at an exit side, respectively. The complex lens has a plurality of stacked lens portions and the lens portions converging a plurality of light beams, which are modulated independently and deflected by a deflector, onto a surface to be scanned, respectively, for forming a plurality of scanning lines at the same time.