The invention provides a semiconductor display device with less generation of a pseudo contour while the drive frequency of a driver circuit is suppressed. Furthermore, the invention provides a semiconductor display device with less generation of a pseudo contour while the decrease in image quality is suppressed. A semiconductor display device comprises a table storing data for determining a relationship between the gray scale level of a video signal and a subframe period for light emission in the plurality of subframe periods, a controller for changing a video signal in accordance with the data and outputting, and a panel whose pixel gray scale level is controlled in accordance with the outputted video signal. The number and the length of the plural subframe periods for each gray scale level of 2 or more are determined in accordance with a subframe ratio R.sub.SF which is calculated in accordance with a sharing ratio R.sub.sh determined by the frame frequency.