An electrical control unit for an automobile comprising: a microcomputer which controls equipment installed in an automobile, an input circuit which passes an electric signal from the outside to said microcomputer, a driver circuit which outputs the electric signal output from said microcomputer outside, and a power supply circuit which supplies power to said microcomputer, wherein said power supply circuit is a semiconductor IC integrating the following components: a communication driver which transmits and receives the communication signal with an outside electrical control unit through a communications line; a first regulator providing a first power supply to a CPU core (VDDL) of said microcomputer to operate said microcomputer based on an ignition switch signal or at least one wake-up signal received from the outside through said communication driver; a second regulator providing a backup power supply to the components of the microcomputer that require a power supply when the microcomputer is in a sleep state and which operates even when said ignition switch signal is cut off; an OR circuit activating said first regulator by using either the ignition switch signal or the at least one wake-up signal; a start factor determining means to determine a start factor that determines which of the ignition switch signal or the at least one wake-up signal has activated said first regulator, wherein said start factor determining means includes a register which stores said start factor, and said start factor is transmitted from the register to said microcomputer after it is waked up; a latch circuit to latch the wake-up signals transmitted from the outside electrical control unit through said communications line; a reset circuit which generates a reset signal to said microcomputer; a watchdog timer to prevent the runaway of the active program in said microcomputer; and a serial communications module which communicates serially with said microcomputer. a microcomputer which controls equipment installed in an automobile, an input circuit which passes an electric signal from the outside to said microcomputer, a driver circuit which outputs the electric signal output from said microcomputer outside, and a power supply circuit which supplies power to said microcomputer, wherein said power supply circuit is a semiconductor IC integrating the following components: a communication driver which transmits and receives the communication signal with an outside electrical control unit through a communications line; a first regulator providing a first power supply to a CPU core (VDDL) of said microcomputer to operate said microcomputer based on an ignition switch signal or at least one wake-up signal received from the outside through said communication driver; a second regulator providing a backup power supply to the components of the microcomputer that require a power supply when the microcomputer is in a sleep state and which operates even when said ignition switch signal is cut off; an OR circuit activating said first regulator by using either the ignition switch signal or the at least one wake-up signal; a start factor determining means to determine a start factor that determines which of the ignition switch signal or the at least one wake-up signal has activated said first regulator, wherein said start factor determining means includes a register which stores said start factor, and said start factor is transmitted from the register to said microcomputer after it is waked up; a latch circuit to latch the wake-up signals transmitted from the outside electrical control unit through said communications line; a reset circuit which generates a reset signal to said microcomputer; a watchdog timer to prevent the runaway of the active program in said microcomputer; and a serial communications module which communicates serially with said microcomputer. a communication driver which transmits and receives the communication signal with an outside electrical control unit through a communications line; a first regulator providing a first power supply to a CPU core (VDDL) of said microcomputer to operate said microcomputer based on an ignition switch signal or at least one wake-up signal received from the outside through said communication driver; a second regulator providing a backup power supply to the components of the microcomputer that require a power supply when the microcomputer is in a sleep state and which operates even when said ignition switch signal is cut off; an OR circuit activating said first regulator by using either the ignition switch signal or the at least one wake-up signal; a start factor determining means to determine a start factor that determines which of the ignition switch signal or the at least one wake-up signal has activated said first regulator, wherein said start factor determining means includes a register which stores said start factor, and said start factor is transmitted from the register to said microcomputer after it is waked up; a latch circuit to latch the wake-up signals transmitted from the outside electrical control unit through said communications line; a reset circuit which generates a reset signal to said microcomputer; a watchdog timer to prevent the runaway of the active program in said microcomputer; and a serial communications module which communicates serially with said microcomputer.