A head band unit contains a bending sensor which detects bending of the head band unit. As shown in the figure, the bending degree of the head band unit when mounted on a user head is smaller than the bending degree of the head band unit when not mounted. Accordingly, the bending degree of the bending sensor is also smaller when mounted. Consequently, by using the output of the bending sensor, it is possible to judge whether the headphone is mounted on the user head. When the headphone is mounted on the user head, power supply to a secondary battery is stopped. Thus, it is possible to provide an output device which can safely charge the secondary battery and can continue operation by an external power source even if the electric capacity of the secondary battery has become zero.