Abstract: This is to present a protecting device for processing tool of machine tool used in a machine tool of a type of machining such as grinding and cutting a work by rotating a processing tool mounted on a rotary spindle, for protecting the processing tool by preventing from damage or breakage due to excessive torque load. In a honing machine, for example, a magnetic coupling device (25) for transmitting the rotating force of a servo motor (16) to a rotary spindle (2) by magnetic action is provided between the rotary spindle (2) on which a honing tool (1) as processing tool is mounted and the servo motor (16) of a spindle rotation driving part (3) for rotating the rotary spindle (2), and the magnetic transmission torque of the magnetic coupling device (25) for transmitting the rotating force is set smaller than a specified torque load applied to the honing tool (1) (for example, breakage torque of honing tool (1)).