To provide an internal gear which can reduce eccentric amount of a center of gravity in a simple constitution and also, can identify a forming method, and a production of the internal gear by sintering, which can identify the forming method.

This internal gear is produced by integrally forming an outside frame part 20 with one tooth part 21 formed along the almost cylindrical inner peripheral surface 20a, an almost annular flange part 30 connecting the outer peripheral edge with one side peripheral end part of this outside frame part 20 and a boss part 40 positioned at the inner ring part of this flange part 30 and having the other tooth part 41 formed along the inner peripheral surface 40a in the inside with a forming die and a sizing die and by sintering. In the flange part 30, a recessed part 37 for matching a tooth-profile relative position of the one tooth part 21 with the other tooth part 41 is formed between the forming die and the sizing die, and also, the recessed part 37 is used for positioning the center of gravity of the internal gear 10 to the center of rotation so as to be commonly used, and is formed at the outside surface. Further, in the production of the internal gear by sintering, when forming the internal gear 10 from a sintered powder, different recessed parts for distinguishing the forming method, is formed in the formed body.