Abstract: A drilling tool comprises a holder unit adapted to be detachably mounted to a hand-held drive unit, and a hole saw unit detachably mounted to the holder unit, the hole saw unit including, as a unit, a cylindrical cutter and a center pin, the holder unit including, as a unit, a spring and a slider disposed to allow reciprocating motion of the center pin, whereby the drilling tool is able to constantly form an appropriate hole in a workpiece such as a thin iron plate, a thick iron plate, or a pipe having a round surface, and expandable components of the drilling tool are formed as a unit to allow for ready replacement.