A ringing suppressor circuit is connected to a differential signal transmission line that includes a high potential signal line and low potential signal line pair for transmitting high and low level differential signals, and includes a ringing suppressor and a stopper. When the differential signal changes to a high level, the ringing suppressor suppresses ringing by lowering the impedance between the signal lines by turning ON of a switching element. When a differential signal voltage drops below a voltage lowering determination voltage, the stopper stops the impedance lowering function of the ringing suppressor by turning OFF another switching element.