To provide a tool breakage detector which is durable under an adverse environment of a coolant, space-saving and inexpensive in linear motion plunger system tool breakage detection.

The tool breakage detector of air-drive type is less failed against the ingress of a cutting agent because no electric motor is used, space-saving and inexpensive because any electric control unit is unnecessary. A plunger 3 having a contact 2 abutted on the tool is connected to a piston of an air cylinder-shaped air drive unit via a coil spring, and the contact force when the contact is abutted on the tool is not dependent on the air pressure, but reduced to the coil spring pressure. Further, a sensor is mounted in a plunger shaft and an inner case, and not exposed outside, and any factor of failure caused by the adverse external environment can be avoided. By press-contacting the plunger against a protective cover provided on the plunger and a flange provided on an outer case 1 via a seal at an end position when the plunger is pressed, a protective structure against the coolant and dust is enhanced.

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