In a wave gear device, similarity-curve tooth profiles BC and AC are adopted as addendum tooth profile shapes for gears, the tooth profiles being derived from a movement trajectory M1 of an external tooth of a flexible externally toothed gear having zero deviation (κ=1), in relation to an internal tooth at a main cross-section. Profile shifting is performed on the external tooth along the tooth trace for a movement trajectory M3 of the external tooth to share a bottom portion with the movement trajectory M1 at the main cross-section from the main cross-section to an inner edge. Profile shifting is performed on the external tooth along the tooth trace for the movement trajectory of the straight-line tooth-profile portion of the external tooth from the main cross-section to the outer edge to be consistent with the movement trajectory of the straight-line tooth-profile portion of the main cross-section.