In order to display an image on a liquid crystal display at a constant image quality independently of a change of the length of the period for applying voltages with the same polarity to the liquid crystal, the length of one cycle of a horizontal synchronizing signal inputted to an LCD unit from an information processor is counted and the amplitude of data voltage and the center of the amplitude are corrected so that the amplitude of the data voltage to be applied to the liquid crystal increases and a correction value for the center of the amplitude increases as the length of the period for applying data voltages with the same polarity to the liquid crystal decreases in accordance with the length of one cycle of the horizontal synchronizing signal and whether to perform gate overlap scan. Thereby, the absolute values (V) of inter-electrode voltages after the TFT is turned off are equalized independently of the polarity of data voltage to be applied to the liquid crystal both for a relatively long period and for a relatively short period.