The present invention is proposed for attaining the above object by supplying a drilling screw comprising a parallel thread part which has a trumpet head at its one end; a taper thread part; a smaller diameter parallel thread part, which has a taper bit at its fore end and is connected to the parallel thread part through a taper thread part; and {TAIL ~ a{HEAD ~  thread crest}}, which passes from the parallel thread part to the smaller diameter parallel thread part and has a decreased height at the above smaller diameter parallel thread part 