A controller controls the driving frequency and voltages for a display device. If image data corresponds to a moving picture, the controller drives a data driver and a gate driver at a moving picture frequency. If image data corresponds to a still image, drives the data driver and the gate driver at a still image frequency lower frequency than the moving picture frequency. When the still image is to be displayed, the signal controller also controls leakage current of a thin film transistor of a pixel based on a representative value of the image data, such that positive leakage current applied for a positive data voltage is equal to negative leakage current applied for a negative data voltage.