Figures 6, 7 and 8 show a moving locus ℓ of one of teeth of the cup-shaped flexible external gear with respect to a tooth groove of the rigid internal gear 2 of the flexing contact type gear drive 1,the moving locus being obtained by the rack approximation provided that the numbers of the both gears 2 and 3 become infinite without changing the difference in number of teeth between the both gears. Each of the obtained moving loci is in a state of negative deviation. Note that in these figures, the teeth of the internal and external gears are provided with same tooth profiles provisionally.