Provided is an electronic control device including a timer that operates during a key-off time period, the electronic control device having high reliability to diagnose whether the timer is normally operated even during the key-off time period. The electronic control device includes a first power supply unit to which a battery voltage is always supplied as a power supply voltage, the first power supply unit including a first timer that measures a key-off time period, a diagnostic timer different from the first timer, and a first timer diagnosis unit that compares a timer value of the first timer with that of the diagnostic timer during the key-off time period.