An emulator dedicated one-chip microcomputer which can emulate without making any malfunction or without being subject to influences of noise even at low voltage, for example, when line voltage is about 1.8V, which is equipped with a level shifter 25 for converting the voltage level of signals given from the internal circuit bus 10 to the output circuit 4 from the voltage level of the external power supply VCC to the voltage level of the emulator power supply VCCA when the external power supply VCC and the emulator power supply VCCA equivalent to the voltage level at which the emulator 2 operates are supplied and an output circuit 4 outputs signals on the internal circuit bus 10 to the emulator 2 via a plurality of signal lines 7, 8, 9, and which is configured to allow the output circuit 4 to operate when the emulator power supply VCCA is supplied.