A semiconductor integrated circuit includes a digital circuit and a first-stage register circuit provided in a stage followed by the digital circuit. The digital circuit includes a logic circuit and a register circuit configured to temporarily retain a logic output from the logic circuit. The first-stage register circuit has a function as an alternative configured to test at least one register circuit and a function as an interface which supplies input data from an external input terminal to the digital circuit. The first-stage register circuit retains the input data from the external input terminal in synchronization with a clock signal, supplies the retained data to the digital circuit at the time of system operation, and outputs the retained data from an external output terminal connected to a dedicated output terminal or the digital circuit at the time of testing.