A metal core for a resin gear and a resin gear composite member are provided. The metal core includes a boss portion into which a shaft is press-fitted, a ring portion to which an outer peripheral resin is joined, and a connecting portion via which the boss portion and the ring portion are coaxially provided. The boss portion is arranged on one side of the connecting portion at an inner circumference of the connecting portion and extends annularly in an axial direction. The ring portion is arranged on the same side of the connecting portion at an outer circumference of the connecting portion and extends annularly in the axial direction. The boss portion, the connecting portion, and the ring portion are formed of an integrally continuous material. The boss portion is thicker than the connecting portion and the ring portion.