In a display device the possibility that the ratio between a display period for display signals and a display period for blanking signals differs from a preset ratio is eliminated, even when the video data is changed. A display device sequentially supplies blanking data from a data driver circuit which sequentially supplies display signals after a lapse of a given time from starting the supply of the display signals. In the display device, a ratio of display is set based on the blanking data per one frame period. Further, the number of pulses of a horizontal synchronizing signal is measured in one frame period contained in the video data, and a point of time is determined for starting display based on the blanking data in response to pulses of the horizontal synchronizing signal corresponding to the ratio based on a measured value of the number of pulses.