To provide a mounting structure of an electric wire assembly easy to be inserted into an insertion hole of a plate member.

In the mounting structure C of the electric wire assembly, a wire harness 19 is retained by a grommet 19d in the insertion hole 49a formed in a barrier rib 49 to partition an engine room 52 and a crew cabin 51 as a compartment. In the wire harness 19, a connector 29 is coupled to the end parts 36a, 39a of electric wire bundles 36, 39. In neighboring parts 36a2, 39a2 of the end parts 36a, 39a as prescribed parts of the electric wire bundles 36, 39 between the grommet 19d and the connector 29, electric wire binders 38, 40 to be commonly used also for a part of a jig for an airtight test are retained. The maximum diameters L1, L3 of the outer peripheral parts 38d, 40d of the electric wire binders 38, 40 are made smaller than the diameter L2 of the insertion hole 49a, and the outer peripheral parts 38d, 40d of the electric wire binders 38, 40 are made to be tapered toward the connector 29 side.