In a wave gear device (1), radial deflection of a flexible external gear (3) is set at 2κmn (κ>1) which is larger than a normal deflection of 2nm (κ=1), and the tooth depth of both gears (2, 3) is set larger than a standard tooth depth (in case of κ=1). Meshing movement locus of both gears (2, 3) is approximated by a rack in case of κ>1, and using similarity-transformation curves (AC, AD) obtained by similarly enlarging a curve (AB) on the unmeshing side at the post-stage of meshing from the vertex (deepest meshing position)(A) of the movement locus (M), the dedendum tooth profile portion (21) of a rigid internal gear (2) and the addendum tooth profile portion (31) of the flexible external gear (3) are defined and then both gears (2, 3) are brought into continuous contact.