Abstract: A controlling unit for a.c. driving an electrostatic device such as an electrostatic display device having a fixed electrode, a resilient sheet electrode and a dielectric layer interposed between the electrodes wherein the sheet electrode is drawn to the outer surface of the fixed electrode to cover the same upon the application of a voltage between the fixed and the sheet electrodes whereas the sheet electrode is kept apart from the fixed electrode upon the removal of the voltage between the electrodes. The controlling unit of this invention includes a resistance in series with an a.c. voltage supply and the electrostatic display device, a triode thyristor switch with a first terminal and a second terminal connected across the fixed and the sheet electrodes, and a switching means for controlling the gate of the thyristor so that the thyristor has an on-off switching function at its leakage current region.