A driver circuit in a high-speed serial communications system changes an input data signal into a differential signal. A first output terminal is connected to a predetermined power supply voltage through a first pullup resistance circuit and to a grounding voltage through a first pulldown resistance circuit. A second output terminal is connected to the predetermined power supply voltage through a second pull-up resistance circuit and to the grounding voltage through a second pulldown resistance circuit. Resistances of the first and second pull-up resistance circuits and resistances of the first and second pulldown resistance circuits are changed according to the input data signal.