The invention provides a lens block having a unitized structure of a plurality of lens elements. A portion or whole of the plurality of lens elements and spacing rings 30 to 38 as constitutional components of an optical system are inserted into a lens barrel type jig 40 in prescribed order. A receptacle 43 which receives the lens element 38 to be inserted first is formed in the jig 40. After respective lens elements 30 to 38 are inserted into the jig 40, the respective lens elements 30 to 38 are made to abut with each other in one direction in an optical axis direction and positioned while fixing the distances related to respective lens elements 30 to 34, 36 and 37. Thereafter, adhesive agent is applied on at least three positions on the outer circumference of the abutting parts where the respective lens elements and spacing rings 30 to 38 abut with each other, and also, in the circumferential direction through openings 41 formed in the outer circumference of the jig 40, then, the plurality of lens elements and spacing rings 30 to 38 are unitized. |