Abstract: A physical layer device 21 includes a link layer interface 2, a physical layer logic circuit 3, and ports 4 to 6. In addition to these components, the physical layer device 21 includes therein a test link layer circuit 22, a test physical layer logic circuit 23, and switches 24 to 26 in order to test the operation of the physical layer logic circuit 3 and the ports 4 to 6. In testing, the ports 4 to 6 are externally connected by a cable 27, and contacts of the switches 24 to 26 are switched. Accordingly, the physical layer logic circuit 3 is connected to the test link layer circuit 22, and the ports 5 and 6 are connected to the test physical layer logic circuit 23. According to the present invention, testing can be performed by a physical layer device alone, thus reducing the test time and the test cost.