The vehicle-installed microcomputer system includes an A/D converter converting analog signal sent from an onboard sensor circuit into digital signal, a microcomputer which operates in one of regular mode where a predetermined arithmetic computation on the digital signal is performed, and sleep mode where the predetermined arithmetic computation is prohibited from being performed, a first power supply circuit supplied with electric power from outside to produce a power supply voltage to be applied to the microcomputer, a second power supply circuit supplied with electric power from outside through an interruption switch to produce a power supply voltage to be applied to the A/D converter and the sensor circuit, the second power supply circuit having an output voltage accuracy higher than that of the first power supply circuit. The power interruption switch is opened to prohibit the second power supply circuit from being supplied with electric power in the sleep mode.