PURPOSE: To automatically keep grounding load to an initial designated grounding load following the waved floor surface when a change of the grounding load is caused for a preset load by storing and grounding a rotary brush by a cylinder.

CONSTITUTION: In a condition where a designated grounding load is produced a brush following member 30 and a rod following member 40 keep a designated relative positional relation. When a rotary brush 7 is moved up and down due to the abrasion of a brush and the waved floor surface, grounding load set springs 26, 27 interposed between a piston rod 15c and the rotary brush 7 are elongated and contracted so that the brush following member 30 is relatively moved to the rod following member 40. The relative movement is detected by a detector 46 and a cylinder 15 is controlled to return the brush following member 30 and the rod following member 40 to the initial designated relative positional relation. Thus, the grounding load is automatically kept to a designated value by returning the grounding load set springs 26, 27 to the initial state.