In driving a motor by a solar cell, if the sun light is weak, the motor cannot be started because of the fact that the start-up current of the motor is more than the current needed for driving. In this invention, a current limiting circuit is placed between the solar cell and the load, whereby the current limitation by said limiting circuit is released when the detected voltage of the solar cell reached the value required for starting the load. The current limiting circuit uses a semiconductor element such as a thyrister and a transistor which general limits a current. Such a current limiting circuit is controlled by a voltage detecting circuit and is so constructed that the cell limitation is released when a specific preset volatge of the solar cell is reached. 2 figs.