A bidirectional torque limiter comprising an input member, an output member, an outer race and an inner race, which share a central axis, wherein an input locking piece extending axially is formed at the input member and an output locking piece extending axially is formed at the output member, the input locking piece and the output locking piece are arranged in combination to form two gaps (39a, 39b) in the circumferential direction, an external coil spring is mounted on the outer peripheral surface of the outer race, the external coil spring in a free state has an inner diameter smaller than the outer diameter of the outer race, and the external coil spring clamps the outer race with a relatively small clamping force, the external coil spring has two external hook portions (44a, 44b) at angular positions different from each other in the circumferential direction, and each of the two external hook portions (44a, 44b) is inserted in each of the two gaps (39a, 39b), an internal coil spring is mounted on the outer peripheral surface of the inner race, the internal coil spring in a free state has an inner diameter smaller than the outer diameter of the inner race, and the internal coil spring clamps the inner race with a relatively large clamping force, the internal coil spring has two internal hook portions (50a, 50b) at angular positions different from each other in the circumferential direction, each of the two internal hook portions (50a, 50b) is inserted in a hook groove formed on the inner peripheral surface of the outer race, or each of the two internal hook portions (50a, 50b) are arranged at the both sides of a hook ridge formed on the inner peripheral surface of the outer race, when a rotation torque is applied to the input member, the input locking piece pushes one of the two external hook portions (44a, 44b) so as to increase the diameter of the external coil spring, and when a rotation torque is applied to the output member, the output locking piece pushes one of the two external hook portions (44a, 44b) so as to decrease the diameter of the external coil spring and to clamp the outer race, the outer race pushes the other internal hook portion so as to increase the diameter of the internal coil spring. an input locking piece extending axially is formed at the input member and an output locking piece extending axially is formed at the output member, the input locking piece and the output locking piece are arranged in combination to form two gaps (39a, 39b) in the circumferential direction, an external coil spring is mounted on the outer peripheral surface of the outer race, the external coil spring in a free state has an inner diameter smaller than the outer diameter of the outer race, and the external coil spring clamps the outer race with a relatively small clamping force, the external coil spring has two external hook portions (44a, 44b) at angular positions different from each other in the circumferential direction, and each of the two external hook portions (44a, 44b) is inserted in each of the two gaps (39a, 39b), an internal coil spring is mounted on the outer peripheral surface of the inner race, the internal coil spring in a free state has an inner diameter smaller than the outer diameter of the inner race, and the internal coil spring clamps the inner race with a relatively large clamping force, the internal coil spring has two internal hook portions (50a, 50b) at angular positions different from each other in the circumferential direction, each of the two internal hook portions (50a, 50b) is inserted in a hook groove formed on the inner peripheral surface of the outer race, or each of the two internal hook portions (50a, 50b) are arranged at the both sides of a hook ridge formed on the inner peripheral surface of the outer race, when a rotation torque is applied to the input member, the input locking piece pushes one of the two external hook portions (44a, 44b) so as to increase the diameter of the external coil spring, and when a rotation torque is applied to the output member, the output locking piece pushes one of the two external hook portions (44a, 44b) so as to decrease the diameter of the external coil spring and to clamp the outer race, the outer race pushes the other internal hook portion so as to increase the diameter of the internal coil spring.