A spiral tap has a male thread disposed on an outer circumferential portion and a cutting edge formed along a spiral flute disposed spirally around an axial direction so as to divide the male thread, the spiral tap is disposed with a sub-groove formed into a concave shape along a back edge of the spiral flute to make a rake angle of the back edge positive at least in a portion corresponding to a biting portion of the spiral tap in the spiral flute, and a curvature radius of the sub-groove is smaller than a curvature radius of the spiral flute in a cross section perpendicular to the axial direction.