Provided is an in-vehicle electronic control device that can properly diagnose the specifics of a microcomputer abnormality. A monitoring function (23) for detecting a failure by monitoring the input and output of a main function (22) of a hardware unit (21) and a monitoring function (26) for detecting an abnormality by monitoring computation results from a main function (25) of a software unit (24) are provided to the interior of a microcomputer (MCU (2)). The main functions being monitored and the monitoring functions for failures and abnormalities are installed with different structures. Also, a failure processing circuit (5) that monitors abnormalities in the microcomputer (MCU (2)) is provided to the exterior of the microcomputer (MCU (2)).