An object of the present invention is to provide a power generating device which makes it possible to easily generate electric power in a rotating system, such as a wheel including a tire, without impairing the performance and reliability of the rotating system. In the case where the power generating device is mounted to a rotating body such as a tire, the direction of gravity applied to a weight is changed with the rotation of the rotating body to cause beams to be deflected, so that the positions of the deflected beams and the weight are fluctuated in synchronization with the period of the rotation to cause the position of a coil to be displaced. Thereby, the weight is vibrated in the vertical direction in synchronization with the period of the rotation of the rotating body to change the magnetic flux density crossing the coil. As a result, AC electromotive forces are generated in the coil to enable electric power to be generated.
