Tendon driven mechanism is one of the most popular mechanism for transmitting force and power from a distance. The energy efficiency of a tendon driven system can be improved if it can maintain actuation force while it is not moving without mechanical work. This could be achieved by a brake; a brake without an additional actuator is preferred for the compactness of the whole system. We present a novel passive brake mechanism, a capstan brake, which consists of a capstan and two one-way clutches. The friction between the capstan and the cable amplifies a small resisting force (originated from an inactive motor) to gain enough brake force. Because no additional actuator is involved, generation of the brake force does not consume energy. Also, the one-way clutches enable the capstan to rotate in the winding direction. Therefore, the brake force is exerted only when it is needed, and the performance of the whole device does not decrease owing to the use of the capstan brake. The performance of the proposed brake mechanism has been evaluated through several tests. The results show that the amount of the maximum brake force for the test condition is more than 55 N (and can be further increased by increasing the number of windings), and that the force loss from the brake is negligible.