A USB test circuit for use in a USB device such as a system LSI with a USB function for testing the USB function generates and outputs a packet to be measured for a signal quality test. In the test circuit, a test signal including a test_sin signal carrying operation mode information is inputted via a serial interface to a serial interface block, and a packet to be measured is generated by a data pattern generation block and a transmission data delivery block depending on the operation mode information. The packet to be measured is outputted via a UTMI interface to a USB PHY layer. Thus, a packet to be measured for a signal quality test is generated and outputted without receiving packets not to be measured such as a SETUP packet and a DATA packet, thereby reducing the test time.