An electronic control device includes a relay including a relay case and an electromagnet and a contact part. The relay case is structured to contain the electromagnet and the contact part. The contact part is structured to be opened and closed by action of the electromagnet. A connector terminal is electrically connected to the relay. A circuit board is structured to mount the relay and the connector terminal thereon. An exterior shell is structured to contain the circuit board such that the connector terminal is exposed outside the exterior shell. A thermal mass element is disposed in a wiring route between the relay and the connector terminal.