Each of an external tooth-bottom surface and an external bottom-side contact surface has a cross-sectional shape formed by a hypocycloid curved line. An external front-side contact surface has a cross-sectional shape formed by an epicycloid curved line. An external tooth-front surface is formed at a radial-inside position of a reference epicycloid curved line, that is, a position closer to a first pitch circle of an external gear. A possible contact between a tooth-front portion of the external gear and a tooth-front portion of an internal gear can be avoided. A connecting point between the external tooth-front surface and the external front-side contact surface is located at such a position that a contact surface length of an external tooth-contact surface is larger than twenty percent of a total tooth surface of the external gear.