A semiconductor device includes an output path; an input path; and a test signal generating circuit. The test signal generating circuit generates an input test data signal by changing at least one of an amplitude and a phase of an output test data signal which is generated from a test data in the semiconductor device and transferred on the output path, and supplies the input test data signal onto the input path. The output path and the input path are tested by using the output test data signal and the input test data signal, respectively.