wherein the opening (3a) of the flexible external gear (3) is set in a state of negative deviation wherein a deflection of the opening (3a) is lower in value than a normal deflection; a moving locus (I) is obtained through rack approximation of teeth (30) of the external gear (3) to the internal gear (2) at respective sections of rotation of tooth trace of the flexible external gear (3); an envelope (e) is obtained by overlapping the moving locus (I) on one of sections of rotation; another moving locus (1g) is obtained in a section of rotation at an end portion of tooth trace on a side of the diaphragm (3b) or in the vicinity of an outside of the end portion and is smoothly connected to the envelope (e) to thereby obtain a composite curve (L); a curve (FE) is obtained by similarity transforming at a reduction ratio of 14 a portion from a starting point (A) of the envelope (e) to a terminating point (E) thereof corresponding to two times the working depth in the direction of the depth from an apex of a moving locus (lo) when deviation is 0, using the terminating point (E) as an original point; and this curve (FE) is made to be each of main portions of convex tooth profiles at tooth faces of the both gears (2, 3); and,wherein a composite curve (MFE) including a straight line (MF) having a limited pressure angle (αM)associated with a deflection value of the opening (3a) inserted in the vicinity of a datum point (M) of tooth profile and a transient curve smoothly connecting the straight line (MF) to the main portion of the convex tooth profile, or a similar curve (MFE) thereof is made to be each of tooth profiles at the tooth faces of the both gears (2, 3).