Abstract: In a meshing type gear device (1), spur gears are used for both a rigid internal gear (2) and a flexible external gear (3), and the number of teeth of the flexible gear (3) is made greater by two than that of the rigid internal gear (2). The tooth profile of the flexible external gear (3) is made convex and the shape thereof is that of a curve AC obtained by subjecting a gear tooth portion (curve portion between A and B), which, at the moving path L that the external gear describes relative to the internal gear, as determined by rack approximation, is convex relative to the internal gear, to similarity transformation at a reduction ratio .lambda. using the limiting point A of the contact between the two gears on the path as the origin, while the tooth profile of the rigid internal gear is made concave and the shape thereof is that of a curve AD obtained by subjecting the same portion convex relative to the internal gear at the moving path to similarity transformation at an enlargement ratio (1+.lambda.