According to one embodiment of the present invention, a flat display device which makes drive voltage polarity of pixels of adjacent lines reversed polarity, while drives to reverse drive voltage polarity of the same line on a frame-by-frame basis, has common voltage generation circuit for supplying a common voltage signal to a facing electrode of the flat display device and a control circuit for generating a common voltage control signal supplied to the common voltage generation circuit. The control circuit obtains a control signal for generating common voltage whose average DC potential does not vary, by using a horizontal synchronization timing signal, a vertical synchronization timing signal, and a clock signal. In order to obtain the control signal, the device has an (fh/2) signal generation circuit, an (fv/2) signal generation circuit, an (fh×n) signal generation circuit, an multiplication circuit, a selection control circuit, and a selection circuit.