PURPOSE:To easily detect the commutation state of a DC rotary machine, by a method wherein spark image generated on the sliding face between the commutator and the brush of the DC rotary machine is made incident at the incident end of an optical guide, and wherein spark image radiated from a radiating end is optically received, and wherein the image is converted to an electrical level. CONSTITUTION:The incident end 3 of an optical fiber 2 is just cut off on the end face and is not provided with a micro-lens or the like in condensing system in particular, and the whole range of the sliding face 6 between a brush 4 and a commutator 5 is arranged to included in the angle of incidence at the incident end 3 of the optical fiber, and a light receiving element 7 fitted at the radiating end of the optical fiber 2 is arranged to be able to detect the most of the light wavelength range of the spark image. The spark image received from the radiating end of the optical fiber 2 is converted to an electrical level by the element 7 and is amplified by an amplifier 8, and the input to an arithmetic and logic unit 9 is provided. The pulse quantity of the spark at every given time, spark duration, and an instantaneous value or an average value on an electrical level are detected by the arithmetic and logic unit 9.
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