A transmission probe comprising: an optical element comprising: a main body having a semicircular plate shape, formed of a light transmissive material and including an arc-shaped optical path, wherein irradiation light can travel along the arc-shaped optical path of the circumference of the main body , while repeating total reflection inside the semi-circumference of the main body ; and a gap formed on the arc-shaped optical path in the main body , wherein the gap is shaped into a through groove which penetrates through to a straight portion of the main body , thereby dividing the main body of the optical element into two separate parts, and the transmission probe further comprising: an irradiation purpose optical transmission path configured to emit irradiation light to the optical element such that total reflection occurs on an inner surface of the arc-shaped optical path of the optical element ; and a measurement purpose optical transmission path configured to receive, as measurement light, return light passing through the gap of the optical element and emitted from the arc-shaped optical path. an optical element comprising: a main body having a semicircular plate shape, formed of a light transmissive material and including an arc-shaped optical path, wherein irradiation light can travel along the arc-shaped optical path of the circumference of the main body , while repeating total reflection inside the semi-circumference of the main body ; and a gap formed on the arc-shaped optical path in the main body , wherein the gap is shaped into a through groove which penetrates through to a straight portion of the main body , thereby dividing the main body of the optical element into two separate parts, and a main body having a semicircular plate shape, formed of a light transmissive material and including an arc-shaped optical path, wherein irradiation light can travel along the arc-shaped optical path of the circumference of the main body , while repeating total reflection inside the semi-circumference of the main body ; and a gap formed on the arc-shaped optical path in the main body , wherein the gap is shaped into a through groove which penetrates through to a straight portion of the main body , thereby dividing the main body of the optical element into two separate parts, and the transmission probe further comprising: an irradiation purpose optical transmission path configured to emit irradiation light to the optical element such that total reflection occurs on an inner surface of the arc-shaped optical path of the optical element ; and a measurement purpose optical transmission path configured to receive, as measurement light, return light passing through the gap of the optical element and emitted from the arc-shaped optical path. an irradiation purpose optical transmission path configured to emit irradiation light to the optical element such that total reflection occurs on an inner surface of the arc-shaped optical path of the optical element ; and a measurement purpose optical transmission path configured to receive, as measurement light, return light passing through the gap of the optical element and emitted from the arc-shaped optical path. <EOS>