To prevent the damage of the skin under a cast by a plaster cutting tool by constituting a flat planar base part having an opening window part of a soft resin or rubber and cutting a cast base body along an outer peripheral wall part erected on the circumference of an aperture, thereby forming the plaster window part.

This auxiliary implement 1A for forming the cast window part has the planar base part 11 made of polyethylene having the opening window part 13, the outer peripheral wall part 12 erected on the circumference of the aperture 13 and pins 15 for cutting marks. The planar base part 11 and the outer peripheral wall part 12 have a difference in level of 3mm in height by a difference in their thicknesses. The formation of the window part by the plaster cutting tool 6 is executed by placing the round-saw type cutting edge 61 of the cast cutting tool 6 at the cutting position discriminatable by the pins 15 for cutting marks and a change in the inclination of the cast surface and cutting the cast. Even if the cutting edge 61 cuts the cast 2 and arrives at the auxiliary implement 1A, the cutting to the depth beyond this point is prevented by the flat planar part 11 and the damaging of the skin is averted. The cutting of the lesion 5 is prevented by the outer peripheral wall part 12.