Provided are an automatic impedance adjuster and a control method thereof. The automatic impedance adjuster includes: a pulse signal generating part for generating a pulse signal having a pulse width varied in response to a pulse width control signal; a signal processing part including a plurality of channels having a plurality of first input sensors and a pulse width correction channel having a second input sensor, not outputting the pulse signal when a touch object is in contact with the first and second input sensors, and outputting the pulse signal when the touch object is not in contact with the first and second input sensors; and a controller for outputting a non-contact state when the pulse signal is detected and outputting a contact state when the pulse signal is not detected during a normal mode, entering an automatic impedance adjustment mode to control the pulse width control signal when a pre-determined time elapses, and entering the automatic impedance adjustment mode to control the pulse width control signal when the pulse signal is not detected through all the plurality of channels or the pulse width correction channel. The automatic impedance adjuster includes a specific channel for checking an operating environment. When an operation signal of the specific channel is detected or operation signals of all the channels are detected, the automatic impedance adjustment operation is performed to prevent malfunction of the impedance adjustment operation. In addition, malfunction of a touch sensor due to variation of the operating environment can be prevented to improve operating reliability of the touch sensor.