The purpose of this study was to simplify the design of the tooth surface of face gear and to clarify the optimization method of the specification setting. Face gears have long been used in fishing spinning reels as a speed-increasing gear. Recently, face gears have also been adapted to geared motors for orthogonal reduction gears. However, the design method of the face gear is not supported by a standard theory, and its application is not common. This paper reports the study of face gear that meshes with involute helical gear with offset. The curved surfaces of the face gear tooth can be generated using the involute helical gear surface, which is mesh constant rotation speed. To clarify the meshing of the face gear, the author introduced the idea that the meshing contact progresses along a straight line of action. The meshing point between the involute helical and face gear proceeds at a constant speed along the straight line of action. The straight line of action is continuous and exists as a curved surface. In this method, the meshing contact is always made in the direction normal to the tooth surface of the involute pinion. This method is based on a simple theory and can be used to obtain the tooth surface coordinates of a face gear without complicated repetitive calculations. The tooth profile of the face gear calculated by the conventional generation method using a numerical analysis and calculated by the proposed meshing contact method were compared and, except for the interference area, they were the same. Using this method, design of the tooth surface of a face gear can be simplified.