A gravitational electric power generating device includes a driving shaft, at least a spinning wheel, and two mount assemblies, in which each of the spinning wheels includes a first spoke rim, a second spoke rim, a wheel hub, and at least a tread-wheel. The two mount assemblies are erected at the two ends of the driving shaft, respectively, and can support both the driving shaft and the spinning wheel on the driving shaft. Using a moving object to move in the tread-wheel, and making use of the gravitational effect of the moving object, the spinning wheel is able to drive the driving shaft to rotate. The mount assemblies have a power conversion module, which is connected to an electric generator. The rotational inertia of the driving shaft is transmitted to the electric generator to generate the electric power using the power conversion module.