A pre-emphasis circuit which can improve a communication quality of a data transmission at low cost is provided. A current switch circuit, a current adder circuit, and transition detection circuits are provided in a transmitter of a data transmission system. The transition detection circuits detect transitions of transmission data signals which are a differential pair. The current switch circuit receives the transmission data signals, carries driving currents in accordance with the transmission data signals, and outputs output data signals which are a differential pair. The current adder circuit receives detection signals from the transition detection circuits, and adds driving currents in accordance with the detection signals to load resistors. By this means, output data signals in which the transitions are emphasized are inputted to a transmission line.