To drill holes of plural kind diameters into a thin plate material without replacing a drill, by fitting a reverse conical tubular bush, having a hole diameter to be drilled at lower end, in a drilling position for a work piece, to push a taper drill into the bush until abutting on the bush inner surface.

A taper drill 7 is stopped on a place abutting on the inner surface 8 of a bush 5 to set a hole diameter, consequently the diameter of a hole, to be drilled into a work piece 2, can be selectively drilled by providing only one taper drill 7. The difference, between the diameter D1 of the tip part and the diameter D2 of the diameter-expanded part of the taper drill 7, is 10mm or more, and the tilt of a taper part 10 is 1/10-1/20. This makes a portion wherein a cutting edge is formed have about 60-80mm, thereby making the taper drill 7 have a length easy in handling, and in control drill moving quantity.