To avoid a serious failure and to realize an effective crushing work by judging a load on the cutter of a crusher and selecting an operation speed suitable for the load automatically, or stopping operation automatically in the case of an overload.

A motor driving circuit 2 is connected with a power source 1, and a cutter driving motor 3 is connected with the circuit 2. An electric current detector 4 is connected between the motor 3 and the circuit 2, and a detected value from the detector 4 is input into a control circuit 5. The circuit 5, when a detected value output from the detector 4 exceeds the first prescribed value during the motor 3 being operated in a high speed mode, the motor 3 is shifted into a low speed mode to be operated. When the detected value is the second prescribed value or less during the motor 3 being operated in the low speed mode, the motor 3 is shifted into the high speed mode to be operated, and when the detected value exceeds the third prescribed value during the motor 3 being operated in the low speed mode, the circuit 2 is controlled to stop the motor 3 by operating in a reverse rotation low speed mode.