PURPOSE: To reduce spurious radiation caused by a dot clock which drives a parallel-serial conversion part the display controller which converts parallel display data into serial, display data and sends the data to a display device.

CONSTITUTION: A clock stop circuit 17 decides display data sent from display data memories 11 and 12 to the parallel-serial conversion part 16 and when the data are of all-0 or all-1 constitution, it is decided that the data need not be converted, thereby stopping the operation clock to the parallel-serial conversion part. The operation clock is stopped for gaps between characters, a solid display part, and a part where there is no data change, not to mention the non-display period of characters, figures, so that the spurious radiation caused by the dot clock can be expected to greatly be reduced.