To attain reduction of power consumption in a standing by state, downsizing of a device, and reduction of cost in an electronic control device mounted on a vehicle.

In the electronic control device 1, one watchdog timer circuit 20 carries out both of abnormality observation of a CPU 10 in an operating condition and abnormality observation of the CPU 10 in the standing by state. The CPU 10 in the standing by state carries out a periodical wakeup processing temporarily switching an operation mode from an energy saving mode to a normal mode in a predetermined wakeup period. The wakeup period is shorter than a reset observing time (a time up time) of the watchdog timer circuit 20. A processing checking the state change of a sensor and a processing clearing the counter of the watchdog timer circuit 20 are additionally carried out by the CPU 10 in the standing by state every time when the operation mode is switched to the normal mode.