The lens barrel with an adjustable focus comprises an external mounting unit, an optical unit and a control unit including a focusing motor and the like. The external mounting unit comprises a fixed frame covering the optical unit, a distance ring, an outer circumferential cover, a zoom ring, a back end cover. The optical unit contains a plurality of frame member units that respectively support a plurality of lens groups. Furthermore, after the unitized optical unit is inserted into the fixed frame, the distance ring, outer circumferential cover, zoom ring, back end cover and the like are successively engaged with the outer circumferential part of the fixed frame from the rear side, so that these parts are easily assembled.