PURPOSE: To simplify an optical system, by arranging optical fiber for infrared region between a light emitting laser and a silicon wafer, emitting a laser beam, and transmitting through silicon wafer.

CONSTITUTION: In the measuring device, the laser light emitted from the PbSnTe semiconductor laser 1 which is enclosed in a Dewar vessel 12 is inputted to the light inputed end of the optical fiber 13 for the infrared region. In the meantime, the light emitting end of the optical fiber 13 is located as close as possible to the silicon wafer the vertically with respect to the silicon wafer, and the light is emitted to the silicon wafer. The laser beam is confined within the optical fiber during the transmission but emitted with an emitting angle of 10°W20° at the emitting end and tramsmitted through the silicon wafer 5. Therefore, the combination of a plurality of lenses can be omitted and the optical system can be simplified.