Provided are an optical system, an imaging device, an endoscope system, and a distance measuring system that are small in size and have a wide observation angle of view and are capable of forming an image with parallax for observing an object plane from a plurality of baseline directions. The optical system (1) includes a rotationally symmetric front group (Gf) disposed on a single rotational symmetry axis (C) passing through the center of the image plane (Im), a rotationally symmetric rear group (Gb), An aperture (S), the front group (Gf) has two internal reflection surfaces and two transmission surfaces, and a light beam incident on the front group (Gf) from at least one object surface (Ob) is: An optical path that is incident on the first transmission surface (11), reflected by the first reflection surface (12), reflected by the second reflection surface (13), and emitted from the second transmission surface (14) is formed. Passes through (Gb) and the aperture (S), and forms an image at a position away from the rotational symmetry axis (C) of the image plane (Im) without intermediate imaging within the cross section including the rotational symmetry axis (C). It is characterized by that.小型で観察画角が広く、物体面を複数の基線方向から観察する視差のある像を結像することが可能な光学系、撮像装置、内視鏡システム、及び測距離システムを提供する。 光学系（１）は、像面（Ｉｍ）の中心を通る単一の回転対称軸（Ｃ）上に配置された回転対称な前群（Ｇｆ）と、回転対称な後群（Ｇｂ）と、開口（Ｓ）と、を備え、前群（Ｇｆ）は、２つの内部反射面と２つの透過面を有し、少なくとも一つの物体面（Ｏｂ）から前群（Ｇｆ）に入射する光束は、第１透過面（１１）に入射し、第１反射面（１２）で反射し、第２反射面（１３）で反射し、第２透過面（１４）から射出する光路を形成し、後群（Ｇｂ）と開口（Ｓ）を通過し、回転対称軸（Ｃ）を含む断面内で中間結像すること無く、像面（Ｉｍ）の回転対称軸（Ｃ）から離れた位置に結像することを特徴とする。
