To provide the car body mounting structure of a synthetic resin molded product firmly mounted in the narrow space of a car body without marring decorative appearance.

A mounting part 4 having thickness thicker than that of a parison is formed integrally with a part of an air spoiler 1 molded by blow- molding. A mounting metal fitting 10 is fixed at the mounting part 4. A lock means 17 is inserted through through-holes 7 and 12, formed in the mounting part 4 and the mounting metal fitting 10, to the rear hatch door 5 side, and the air spoiler 1 is fixed at the rear hatch door 5 by the locking means 17.