A reflected light measurement device comprising: a laser light source that emits laser light; a beam splitter that branches the laser light into measurement laser light (L1) to be transmitted and reference laser light (L2) to be reflected; a connection unit that is disposed on an optical path of the measurement laser light (L1) that is transmitted through the beam splitter ; a reference mirror including an optical path length varying mechanism capable of adjusting an optical path length of the reference laser light (L2); and a photometer that receives the measurement laser light (L1) reflected at a defect site of a material to be measured which is formed from a light-transmitting material, and the reference laser light (L2) reflected by the reference mirror through the beam splitter , wherein the defect site is detected on the basis of interference light by the reference laser light (L2) and the measurement laser light (L1) which are received by the photometer , characterised in that the reflected light measurement device further comprises an optical path switching unit that switches an optical path of the measurement laser light (L1) and an optical path length adjustment unit that is connected to the optical path switching unit for adjusting an optical path length of the optical path switching unit are disposed between the beam splitter and the connection unit , wherein the optical path switching unit includes an optical switch unit that performs a switching process, and a plurality of switching optical paths (9b1 to 9bn) capable of being connected to the optical switch unit , the optical path length adjustment unit includes adjustment optical paths (10a1 to 10an) which are connected with the switching optical paths (9b1 to 9bn) in a one-to-one relationship, and each total optical path comprising one of the switching optical paths (9b1 to 9bn) and one of the adjustment optical paths (10a1 to 10an) connected therewith is equal in optical path length. a laser light source that emits laser light; a beam splitter that branches the laser light into measurement laser light (L1) to be transmitted and reference laser light (L2) to be reflected; a connection unit that is disposed on an optical path of the measurement laser light (L1) that is transmitted through the beam splitter ; a reference mirror including an optical path length varying mechanism capable of adjusting an optical path length of the reference laser light (L2); and a photometer that receives the measurement laser light (L1) reflected at a defect site of a material to be measured which is formed from a light-transmitting material, and the reference laser light (L2) reflected by the reference mirror through the beam splitter , wherein the defect site is detected on the basis of interference light by the reference laser light (L2) and the measurement laser light (L1) which are received by the photometer , characterised in that the reflected light measurement device further comprises an optical path switching unit that switches an optical path of the measurement laser light (L1) and an optical path length adjustment unit that is connected to the optical path switching unit for adjusting an optical path length of the optical path switching unit are disposed between the beam splitter and the connection unit , wherein the optical path switching unit includes an optical switch unit that performs a switching process, and a plurality of switching optical paths (9b1 to 9bn) capable of being connected to the optical switch unit , the optical path length adjustment unit includes adjustment optical paths (10a1 to 10an) which are connected with the switching optical paths (9b1 to 9bn) in a one-to-one relationship, and each total optical path comprising one of the switching optical paths (9b1 to 9bn) and one of the adjustment optical paths (10a1 to 10an) connected therewith is equal in optical path length. <EOS>