A signal transmission circuit is capable of reducing distortion that occurs during signal transmission. A digital output signal is transmitted from a terminal to a signal line via an output buffer circuit and an output impedance unit. The terminal is connected to an impedance variation unit via an impedance control unit. The impedance variation unit designates an impedance for terminating the signal line when the output data is changed from a high level H to a low level. A reflection occurring on the signal line can be prevented and a waveform distortion can be suppressed.