PROBLEM TO BE SOLVED: To provide an optical element attachable to the existing optical system, and forming two optical paths of the direct viewing and the lateral viewing and capable of simultaneously imaging the front and all surrounding directions.SOLUTION: An optical element is formed of a rotation symmetrical transparent medium La around the central axis 2. The transparent medium La includes: a first transmission surface 11, a first reflection surface 12 arranged on a center axis 2 of the first transmission surface 11 a second reflection surface 13 arranged opposite to the image surface 5 of the first reflection surface 12 and a second transmission surface 14 arranged at the image surface 5 of the second reflection surface 13. A through-hole Lb is formed at a position including the central axis 2 of the transparent medium La. Light incident on the transparent medium La passes through a side viewing optical path A and a direct-viewing optical path B. The side viewing optical path A, in the order of forward ray tracing, enters the transparent medium La through the first transmission surface 11, and is reflected by the first reflection surface 12 disposed opposite to the image surface 5 and reflected by the second reflection surface 13 to the image surface 5, and constitutes a substantially Z-shaped optical path going to the outside from the transparent medium La to the image surface 5 through a second transmission surface 14. The direct-viewing optical path B constitutes an optical path penetrating a through-hole Lb.SELECTED DRAWING: Figure 1COPYRIGHT: (C)2016,JPO&INPIT【課題】既存の光学系に付加可能で、直視、側視の2光路を形成し、前方と周囲全方位を同時撮像できる光学素子を提供する。【解決手段】光学素子は、中心軸2の周りで回転対称な透明媒体Laからなり、透明媒体Laは、第1透過面11と、第1透過面11より中心軸2側に配置された第1反射面12と、第1反射面12より像面5と反対側に配置された第2反射面13と、第2反射面13より像面5側に配置された第2透過面14とを有し、透明媒体Laの中心軸2を含む位置に貫通孔Lbが形成され、透明媒体Laに入射する光は、側視光路Aと直視光路Bとを通過し、順光線追跡の順に、側視光路Aは、第1透過面11を経て透明媒体La内に入り、第1反射面12で像面5と反対側に反射され、第2反射面13で像面5側に反射され、第2透過面14を経て透明媒体Laから像面5側に外へ出る略Z字状の光路を構成し、直視光路Bは、貫通孔Lbを貫通する光路を構成する。【選
