A cutter fracture detecting system is capable of detecting a fracture of a cutter highly accurately which may occur even when no substantial loads are imposed on a rotating axis and a cutter feed axis of a machine tool. A rotating axis disturbant load torque estimating unit (1) and a moving axis disturbant load torque estimating unit (2) monitor disturbant load torques on a spindle and an axis with respect to which a workpiece and a cutter move relatively to each other, and still axis disturbant load torque estimating units (3, 4) also monitor disturbant load torques on axes with respect to which the workpiece and the cutter do not move relatively to each other. A comparing unit (5) determines that the cutter is fractured based on the disturbant load torques estimated by the disturbant load torque estimating units. When a cutter fracture is detected, the machine tool is stopped by a machine shutdown command unit (6).