A resin gear comprises a rim in an outer peripheral part of a plate, teeth provided on an outer periphery of the rim, a boss provided in a central part of the plate, and a circular rib between the boss and the rim. The resin gear is manufacturable by injection molding that a molten material is injected into a cavity through a plurality of gates arranged at almost equally spaced intervals circumferentially on a forming portion for the circular rib in a direction perpendicular to a side of the plate. The resin gear includes a plurality of inner thin-wall portions on the plate between the circular rib and the boss and a plurality of outer thin-wall portions on the plate between the circular rib and the rim, each of the inner thin-wall portions and the outer thin-wall portions being symmetrically shaped with respect to the radial line passing through each gate corresponding point.