To provide a mounting method and a mounting structure for a reinforcing connection plate linking a wheel house to a back door pillar for increasing rigidity of a wheel house part and increasing durability of a vehicle body by merely adding the reinforcing connection plate without requiring large-scale change of design.

Both side fringe parts 51, 52 of the reinforcing connection plate 5 like a vertical wall are attached and fixed to a rear part of the wheel house 1 and a pillar inner 21 of the back door pillar 2 by spot welding S2, S3, respectively. An opening part 32 of a drafter is provided in a body side outer panel 3 at a position opposing to a lower end fringe part 55 of the reinforcing connection plate 5. An electrode 61 on one side of a spot gun 6 is inserted through the opening part 32 of the drafter to attach and fix the lower end fringe part 55 of the reinforcing connection plate 5 by spot welding S4 in the vicinity of a connection part with the body side outer panel 3 in a floor panel 4.