A total reflection optical member made of a plate-shaped optical member, comprising: a leading-in part and a leading-out part of a measurement light provided at positions deviated from a center of either surface of front and back surfaces of the optical member; a plurality of plane parts (2B, 2C, 2E) formed perpendicularly to the front and back surfaces of the optical member respectively at an outer periphery other than the front and back surfaces of the optical member, wherein the leading-in part is provided to make the measurement light that is configured to be led inside the optical member incident toward either surface of the front and back surfaces of the optical member at an angle of incidence of total reflection, the front and back surfaces of the optical member are provided to make the measurement light travel while totally reflecting the same alternately, the plurality of plane parts (2B, 2C, 2E) is provided to sequentially reflect the measurement light that is configured to travel inside the optical member in a different direction, and the leading-out part is configured to lead out the measurement light that is reflected at the plurality of plane parts (2B, 2C, 2E) to outside, characterized in that among the plurality of plane parts (2B, 2C, 2E), the plane part that reflects the measurement light secondary or after is provided so that an optical path of the measurement light that is reflected at the plane part is configured to cross the optical path of the measurement light that traveled toward the other plane part that has reflected the measurement light precedingly when the optical paths are projected onto the planes of the front and back surfaces of the optical member. a leading-in part and a leading-out part of a measurement light provided at positions deviated from a center of either surface of front and back surfaces of the optical member; a plurality of plane parts (2B, 2C, 2E) formed perpendicularly to the front and back surfaces of the optical member respectively at an outer periphery other than the front and back surfaces of the optical member, wherein the leading-in part is provided to make the measurement light that is configured to be led inside the optical member incident toward either surface of the front and back surfaces of the optical member at an angle of incidence of total reflection, the front and back surfaces of the optical member are provided to make the measurement light travel while totally reflecting the same alternately, the plurality of plane parts (2B, 2C, 2E) is provided to sequentially reflect the measurement light that is configured to travel inside the optical member in a different direction, and the leading-out part is configured to lead out the measurement light that is reflected at the plurality of plane parts (2B, 2C, 2E) to outside, characterized in that among the plurality of plane parts (2B, 2C, 2E), the plane part that reflects the measurement light secondary or after is provided so that an optical path of the measurement light that is reflected at the plane part is configured to cross the optical path of the measurement light that traveled toward the other plane part that has reflected the measurement light precedingly when the optical paths are projected onto the planes of the front and back surfaces of the optical member.