Electronic Control Device and Assembly Method Thereof

Provided is an electronic control device that can reduce a usage amount of an adhesive constituting a seal member, by relieving a stress caused in the seal member disposed in a gap between a first housing part and a second housing part in a high-temperature environment. In an electronic control device including: a component mounting board mounted with an electronic component; and a first housing part and a second housing part that are fixed to each other via a seal member and define a first space that accommodates the component mounting board, at least one predetermined housing part of the first or second housing part has a recess formed on a surface facing the seal member, and a second space is formed between the recess and the seal member.