To provide a machining tool protection device for preventing a machining tool from being damaged and broken due to an excessive torque load in a machine tool of such a type that machining such as grinding and cutting are applied to a work by rotating the machining tool attached to a rotating spindle.

A honing tool 1 which transmits the torque of a servo-motor 16 to the rotating spindle 2 by a magnetic action is installed between the rotating spindle 2 to which a honing tool 1 as a machining tool is attached and the servo-motor 16 of a spindle rotating drive part 3 for rotating the rotating spindle 2. A magnetic transmission torque for transmitting the toque of a magnetic coupling device 25 is set smaller than a predetermined torque load (for example, breakage torque of a honing tool 1) applied to the honing tool 1. Consequently, the damage and breakage of the honing tool 1 by an excessive torque load can be effectively prevented for protection.