An apparatus for molding a resin product having a bottom plate, a pair of side plates, and a pair of flanges. The side plates are formed integrally with the bottom plate to extend from opposite sides of the bottom plate. Each flange is formed integrally with the associated side plate to extend inward from an upper end of the side plate. The apparatus has a pair of inner slidable cores that shape an upper side of the bottom plate, an inner side of the side plate, and a lower side of the flange. The apparatus also includes an intermediate core located between the inner slidable cores for determining the position of each inner slidable core. The apparatus further includes a sliding mechanism that slides each inner slidable core to a position spaced from the resin product while moving the inner slidable core and the intermediate relative to each other.