Resin vehicle part manufacturing method and resin vehicle part

A method for manufacturing a resin vehicle part provided with a panel body having a front surface including a design surface and a stepped reinforcing rib formed standing on a back surface of the panel body, the reinforcing rib having a base end portion with a thickness thinner than a leading end portion,

• wherein an injection molding die for the resin vehicle part includes a first slide piece for forming one stepped reinforcing rib so that the first slide piece is movable in a demolding direction and a second slide piece for forming the other stepped reinforcing rib so that the second slide piece is movable in a direction different from the demolding direction, after molten resin is filled in the injection molding die, the first slide piece and the second slide piece are pushed by the leading end portions of the stepped reinforcing ribs in a direction to come close to the panel body when the stepped reinforcing ribs shrink in a height direction.