Abstract: A microcomputer incorporates an EPROM for storing programs and data which a CPU executes, and includes, as operating modes, a microcomputer mode for operating as a microcomputer, a test mode for testing the operation of the CPU and an EPROM mode for writing in and reading from the EPROM. A test terminal of the microcomputer is used for mode switching control, i.e., used as a power supply testing/programming terminal. By the logic level of this terminal, switching between the microcomputer mode and the test mode is performed, and a programming voltage is also supplied. By a control terminal, which is another terminal, switching between the test mode and the EPROM mode is performed.