Pre-emphasis automatic adjusting method and data transmission system using same

A signal transmission system is provided which is capable of simplifying circuits and shortening time required for automatic adjustment of pre-emphasis. A signal having a single pulse pattern is generated by a single pulse pattern generating circuit. A signal having passed through a selector is divided into two signals whose phases are inverted and which are transmitted to a receiving circuit. An eye aperture of an eye waveform in a direction of time is measured by an eye aperture judging section by using the two signals and a sampling clock output from a sampling clock controlling section. An adjustment controlling section compares the measured eye aperture with a target value for judgment and transmits the judgment result to the transmitting circuit, where pre-emphasis is adjusted based on the judgment result.