An endoscope provided with a mechanism capable of easily and accurately aligning an objective optical system and an imaging optical system, and a method of centering an endoscope. The endoscope 1 includes an objective lens 32 fixed to an observation window 22 provided in a distal end portion main body 20, and an imaging lens 36 a which focuses light incident from the outside of the distal end portion main body 20 via the objective lens 32, , And a ring-shaped member having a circular outer periphery. At least a part of the image pickup optical unit 36 on the inner periphery decenter the optical axis of the image pickup unit 36 with respect to the center axis of the outer periphery , An eccentric ring 40 having an elongated hole shape whose diameter in the minor axis direction is equal to the outer diameter of the eccentric ring 40 is provided and an eccentric ring 40 is provided in the through hole so that the eccentric ring 40 And an image pickup unit housing portion 26 for fitting the outer peripheral surface as a fitting surface.対物光学系と撮像光学系との芯出しを容易且つ精度良く行うことができる機構を備えた内視鏡、及び内視鏡の芯出し方法を提供する。内視鏡1は、先端部本体20に設けられた観察窓22に固定された対物レンズ32と、先端部本体20の外部から対物レンズ32を介して入射する光を結像して撮像素子36aに入射させる撮像ユニット36と、外周が円形状をなすリング状部材であって、内周に撮像光学ユニット36の少なくとも一部が、外周の中心軸に対して撮像ユニット36の光軸を偏心させた状態で配置された偏心リング40と、短軸方向の径が偏心リング40の外径と等しい長孔形状をなす貫通孔が設けられ、該貫通孔に偏心リング40が、該偏心リング40の外周面を嵌合面として嵌合する撮像ユニット収納部26とを備える。
