A cover glass frame as a second frame body mounting thereto a cover glass is fitted onto an objective lens frame, which is a first frame body mounting thereto a tip end lens and an objective lens as optical members, in a manner to be movable in a direction of optical axis, and is fixed after having been adjusted in position in the direction of optical axis. The objective lens frame and the cover glass frame thus fitted and fixed are fitted into and joined to a cylindrical member to constitute an imaging unit, which is then arranged in a tip end portion of an insertion part of an endoscope. Thereby, focus adjustment can be performed on the optical members, which are assembled into the imaging unit, and a tip end side portion of the insertion part of the endoscope can be made small in outer diameter to prevent entry of vapor into the imaging unit even when autoclave sterilization is carried out.