A semiconductor integrated circuit used for in a system for serially transmitting image data as main data, in which audio data is transmitted as sub−data while maintaining the transmission quality without increasing the number of transmission cables. The semiconductor integrated circuit comprises a main data transmitting circuit for converting main data into serial data and transmitting the serial data through at least one transmission line in synchronism with a clock signal and a sub−data transmitting circuit for pulse−width− modulating the clock signal with sub−data, generating a modulated clock signal having at least three pulse widths in a predetermined sequence, and transmitting the modulated clock signal through another transmission line.