In this paper we study the skew gear whose specific sliding is equal to zero. At first the loci of the contact lines are determined in a most general form. Next, aiming at the practical use, such a special case where the contact line is an ordinary helix whose axis is the instantaneous axis is discussed. It is shown that such a gear is a three dimensional extension of the rolling contact gear by Prof. Ishida. The formula of the relative curvature at the meshing point is also given.