The calculation for the deformation of a ring gear was carried out by using the finite element method. The results are as follows: (i) The deflection of the ring gear in the direction of the line of action can be calculated with the deflections of the teeth and the equivalent ring whose stiffness is equal to that of the ring gear. (ii) The expressions are obtained, for the amount of approach between two neighboring teeth, the load sharing factor and the interference of tooth tip with the ring gear. (iii) It is explained theoretically that the contact ratio increases constantly with an increasing gear tooth load. Further, the theoretically obtained result is confirmed by the experimental investigation.