PURPOSE: To detect abrasion amount of brush, regardless of the structure of brush holder or the length of brush, by making a recess in the brush and mounting an optical fiber, responsive to the recess, on the brush holder.

CONSTITUTION: A lead wire 4 of a brush 3, pressed against a collector ring 2 contained in a brush holder 1, is placed on a fixing seat 8 through an intermediate board 7 on the central hole 6 of a lead wire securing ring 5 secured to the brush body, and an optical fiber 9 is fixed to the brush holder 1 by means of fixing bolts 10. When the end of the optical fiber 6 is aligned with the central hole 6 of the lead wire securing ring 5 and the gap between the central hole 6 and the fiber 9 increases, a photoelectric switch functions in a sensor 11 connected with the optical fiber 9. When the abrasion amount of brush 3 is low and thereby the gap is small, the photoelectric switch does not function. According to the constitution, abrasion amount of brush can be detected regardless of the structure of the brush holder or the length of the brush.