PURPOSE: To open a wall hole of an accurate size by providing a ring portion which is installed at the circular hole opening portion of a coupling housing box and a guide hole inserting the center shaft of a hole saw, and providing a projecting cylinder so as to be exposed from the backside toward the surface of a wall member in the center of the ring portion.

CONSTITUTION: Prior to erecting a wall member 90, a prepared hole 92 with a diameter larger than a projecting cylinder 20 while smaller than the circular opening portion 31 of a coupling housing box 30 is opened nearly in the position of a wall member 90 opposite to the circular hole opening portion 31 by means of a hole opening jig 100, and the wall member 90 is erected so that the projecting cylinder 20 is inserted into the prepared hole 92. After that, the center shaft 41 of a hole saw 40 is inserted into a guide hole provided on the projecting cylinder 20 and, by rotating the hole saw 40 of the size conforming to the circular hole opening portion 31 of the coupling housing box 30, a wall hole 91 of an accurate size is provided at the position of the wall member 90 matched to the circular hole opening portion 31 of the coupling housing box 30.