In this paper, cylindrical surfaces, which mesh with each other at one point at a constant angular velocity ratio, are investigated. In the first place, the possibility of this case is investigated, and it is clarified that this is possible when one of a pair is convex and its partner with line contact exists. Then curvatures of these surfaces are made clear. And interference of these gears is studied basing on this result. When one of a pair is given, the partner's generator direction is limited by asymptotic direction of surface which contacts with the given cylindrical surface along the line. Lastly, when one of a pair and the partner's generator direction are given, the theory that the partner's profile is calculated is clarified. These results are the foundation of cutting gears by milling or shaping.