In an in-vehicle electronic control apparatus (ECU), when a microcomputer stops operation, a timer IC starts the count-up operation. When the count value reaches a preset value, a signal from the timer IC becomes high and a power supply voltage is outputted from a power supply IC to activate the microcomputer. The microcomputer reads the count value from the timer IC when it starts the operation. If such a count value has exceeded the specified value, the microcomputer determines occurrence of a failure. Therefore, if the timer IC cannot activate the power supply IC, the microcomputer can detect a failure upon activation with turning on of an ignition switch.