As a prevention method of tooth profile change of a nylon gear, which meshes with a steel gear, this paper proposes a profile modification of steel gear. And a reasonable amount of modification is obtained by experiments. From the results, it is clear that the expression of the amount of profile modification Δα (minutes) of steel gear is as follows. Δα = ζ (Tc·T/9.8b) + ξ ζ : Change value of pressure angle change (minutes)/(N·m/m)·s, Tc : Contact period of teeth of nylon gear (second), T : Torque acting on nylon gear (N·m), b : Face width (m), ξ : Basic value of pressure angle change (minutes).