To perform switching between a maximum revolution number and a previously set low speed revolution number in a single operation.

A switch 18R is turned ON to open a solenoid opening/closing valve 17R, thereby operating a pump 8R. A part of a pressure oil delivered from the pump 8R is made to flow in a bypass circuit 15R, and is returned from a return circuit 13 to a tank 14 via a flow regulation valve 16R. Hydraulic motors 6R and 7R for brush rotation are driven by a remaining pressure oil which is not made to flow in the bypass circuit 15R, and side brushes 4R and 5R are rotated at the set revolution number. When the solenoid opening/closing valve 17R is closed, all amount of oil delivered from the pump 8R is supplied to the hydraulic motors 6R and 7R for brush rotation, and the side brushes 4R and 5R are rotated at the maximum revolution number which is higher than the set revolution number. By turning the switch 18R ON or OFF, the revolution numbers of the side brushes 4R and 5R are switched between the set revolution number and the maximum revolution number.