The side-viewing optical adapter of the present invention has a pair of decentered lenses that have notches on the outer peripheral surface and are arranged side by side so that their optical axes are parallel to each other, and more distal than the pair of decentered lenses And a holding member that has an accommodation hole and holds the eccentric lens, the accommodation hole has a notch receiving surface, and the holding member has a front end opening of the accommodation hole. A prism positioning surface formed at a position closer to the peripheral edge than the notch receiving surface in the case, when viewed from the optical axis direction, at least a part of the notch portions of the pair of decentered lenses is the It is arranged outside the prism in the width direction.本発明の側視光学アダプタは、外周面に切り欠き部を有し、それぞれの光軸が平行となるように並べて配置された一対の偏芯レンズと、前記一対の偏芯レンズよりも先端側に配置されるプリズムと、収容孔を有し前記偏芯レンズを保持する保持部材と、を備え、前記収容孔は、切り欠き受け面を有し、前記保持部材は前記収容孔の先端開口部における前記切り欠き受け面より周縁部に近い位置に形成されるプリズム位置決め面を備え、前記光軸方向からみたときに、前記一対の偏芯レンズのそれぞれの前記切り欠き部の少なくとも一部が前記プリズムよりも幅方向外側に配置される。
