A cathode-ray tube control device is disclosed which displays data obtained from a personal computer on the screen of a television receiver in superimposing relation to the picture of a broadcast. Clock pulses are generated, and these are counted in synchronism with the horizontal and vertical synchronizing signals from the television receiver. Display timing signals including horizontal scanning display period signal, horizontal retrace period signal, vertical scanning display period signal, and vertical retrace period signal are delivered in response to the resultant counts. Then, read-out of data to be displayed is controlled in accordance with the produced timing signals, and the read data is displayed on the screen such that it is superimposed on the picture of a television broadcast.