In a molded plastic part 1 including a body 2 constituting a design surface 2a and a reinforcing structure 3 projectingly provided on a design back surface 2b of the body 2, the body 2 and the reinforcing structure 3 being integrally molded by injection molding, the reinforcing structure 3 is formed by a combined body of a plurality of transverse ribs 7a, 7b, 7c and 7d which are arranged at appropriate intervals in the resin flow direction at the time of injection molding in the direction intersecting with the resin flow and longitudinal ribs 6p and 6q which extend at right angles to the transverse ribs and connect the transverse ribs to each other. The transverse ribs are connected to the design back surface 2b, and a gate 8 for injection molding is set on one of the transverse ribs. The longitudinal ribs are disposed so as to be separated from the design back surface 2b.