In an electronic control unit mounting structure, electronic control units are mounted on a junction block, and printed circuit boards 4a and 4b of the electronic control units, each having a connector 41a, 41b of a through construction mounted thereon, are superposed in such a posture that these connectors 41a and 41b are disposed in registry with each other in an upward-downward direction, and connection terminals 31 of the junction block are inserted in the connectors 41a and 41b.