A screw having a main thread (2) to be screwed in and a sub-thread (3) formed in the direction opposite to the main thread, which is characterized in that a slant surface (3b) of the sub-thread on the screw-loosening side has a sharp slope whereas that (3a) on the screw-tightening side has a gentler slope. When such a tapping screw as above is screwed into a pre-drilled hole (4) and the sub-thread (3) advances in the direction opposite to the screwing-in direction thereof while acting like a wedge because the slope of the slant surface (3a) on the screw-in side is gentler. When the screw is subject to torque in the screw-loosening direction after tightened, the torque acts on the sub-thread (3) in the tightening direction thereof and, because of the sharp slope of the slant surface (3b) on the screw-loosening side, the sub-thread resists the unscrewing of the screw as if eating into the surface of the pre-drilled hole and prevents the screw from loosening.