It is said that helical gears with bias tooth surface modification, a kind of three-dimensional modification, can secure quietness within the wide range of transmitted load, However, its theoretical grounds have not been clarified in the past researches. Helical gears with theoretical involute helicoid have the character that lowers mesh exciting force within the wide range of transmitted load when the transverse or overlap contact ratio becomes an integer. In this paper, the relationship between tooth contact condition and tooth stiffness in such gear pair is discussed. As a result, theoretical grounds for the effectiveness of the proposed three-dimensional modification are clarified. In addition, the way of designing three-dimensional modification to the helical gears that have the arbitrary contact ratio is proposed and the effect is examined by the theoretical analysis.