In a harmonic drive gear device (1), radial deflection of a flexible external gear (3) is set at 2mn (1) which is larger than a normal deflection of 2mn (=1), and the tooth depth of both gears (2, 3) is set larger than a standard tooth depth (in case of =1). Meshing movement locus of both gears (2, 3) is approximated by a rack in case of 1, and using similar 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. Contact pressure of the tooth is lowered by defining the addendum tooth profile portion (24) of the rigid internal gear (2) and the dedendum tooth profile portion (33) of the flexible external gear (3) by a line connected to the similar curves (AC, AD) thus enhancing the torque capacity. A harmonic drive gear device with a tooth profile of low contact pressure having a large tooth depth and a wide meshing region can thereby be attained.
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