A plurality of lens elements and spacer rings 30-38 for constituting a part of an optical system are inserted into a lens-barrel-like jig 40 in predetermined order. A reception portion 43 for receiving the lens element 38 which will be first inserted is formed in the jig 40. When inserted into the jig 40, the lens elements 30-38 are brought into abutment against one another in one way of the optical axis so that spaces among the lens elements 30, 32-34, 36 and 38 are positioned. After that, adhesive is applied to at least three circumferential places of the outer circumference of each abutment portion where the lens elements and spacer rings 30-38 abut against each other, through opening portions 41 provided in the outer circumference of the jig 40. Thus, the plurality of lens elements and spacer rings 30-38 are formed into an integrated structure.