The maximum value of the reaction force generated on the circuit substrate can be reduced.
An electronic control device includes a cover having a hollow opening; a connector inserted into the opening; and a seal member having compressibility that is arranged in contact with a first position of an inner peripheral portion of the opening and that comes into close contact with the inner peripheral portion of the opening and the connector in a compressed state; where the opening has a thickness in a first direction penetrating through the hollow of the opening; an area of a hollow region which is a hollow region of the opening is reduced or maintained along the first direction; the seal member is arranged on an outer peripheral portion of the connector and inserted in the first direction to the opening together with the connector; and an area of the hollow region at the first position is smaller than an area of the hollow region at an entrance of the opening in the first direction.