To provide a method of positively opening an embedded fitting to the surface of an ALC panel while avoiding the contact of the embedded fitting with the edge of a cutter blade of a drilling device when drilling even with dispersion in depth from the surface of the ALC panel to the tip part of the embedded fitting.

In the method of opening the embedded fitting built in an autoclaved lightweight concrete panel, to the surface of the autoclaved lightweight concrete panel, drilling is started from the surface of the autoclaved lightweight concrete panel in the position of the embedded fitting by a drilling means having a drilling load detecting means, and drilling by the drilling means is stopped when the drilling load detecting means detects that the tip of a drilled hole reaches one end of a cavity formed beforehand in the tip part of the embedded fitting, by the decreasing change of drilling load.