To automatically detect the replacement timing of polishing cloth without new investment in a polishing device.

In this polishing device, the replacement timing of the polishing cloth is determined based on a current value and the operating time of a dresser detected in the course of dressing by a motor provided before. The relationship of the motor current average value of a surface plate in the course of dressing in the operating limit of the polishing cloth depending on the accumulated total of the operating time of the dresser is previously found, and when the current value falls below the boundary line, the operating time is determined to exceed the limit, and the replacement timing of the polishing cloth is decided.