In order to quickly and reliably establish link up, when a communication device detects power on or link down, an idle signal generation circuit generates an idle signal. Then, an I/F circuit transmits the idle signal to a communication device which is a communication partner through a selection circuit. Further, the I/F circuit transmits and receives learning signals to and from the communication device which is a communication partner. A Step 1 learning circuit, a Step 2 learning circuit, and a Step 3 learning circuit establish link up by using the learning signals. When not receiving a signal from a link detection circuit indicating that link up is established, a reset mask circuit transmits a reset signal generated by a reset signal generation circuit, to the Step 1 learning circuit, the Step 2 learning circuit, and the Step 3 learning circuit to allow them to learn again.