A rotary current-collecting device includes a rotary slip ring and brushes coming into sliding contact with the outer peripheral surface of the slip ring. The brush-holding ring has an inner surface to which three brush-holding springs are fixed by screws. Each of the brushes is fixed to the tip end of the brush-holding spring and is pushed against the outer surface of the slip ring under the resilient restoration force of the brush-holding spring. When the slip ring revolves, the brushes come into sliding contact with the outer peripheral surface of the slip ring. The brush is made of a metal-graphite compound consisting of 70 weight percent copper and 30 weight percent graphite. The slip ring is entirely made of glassy carbon, so that the brush abrasion can be reduced.