A vehicle-mounted electronic control apparatus can perform a variety of abnormality detections while reducing the control load of a microprocessor in the abnormality detection of a volatile backup memory that is backed up by an on-board battery. The apparatus includes a control CPU which is fed with power from a main power supply circuit upon closure of a power switch, a nonvolatile control memory and a backup memory. Even if the power switch is opened, the backup memory, being a partial area of a RAM memory, is fed with power through an auxiliary power supply circuit connected directly to the battery. When the battery is replaced with a new one and connected again, a power supply interruption monitoring memory is reset. At the start of operation of the control CPU, the backup memory is initialized based on reset information, and the power supply interruption monitoring memory is rewritten into a set state.