PURPOSE: To prevent such a phenomenon that an upper concrete structure lies heavy on a circular saw-shaped blade to obstruct the smooth rotation of the blade, accompanying the advance of cutting when a concrete structure is horizon tal cut by a plunge cutter equipped with the circular saw-shaped blade.

CONSTITUTION: At first, the circular hole 4 coming into contact with a horizontal cutting scheduled surface 3 is provided to the central part of a concrete structure by a drill and plug insertion holes 5 are provided to both sides of the cutting scheduled surface 3 so as to be opened or single side thereof and plugs 6 are preliminarily pushed in the insertion holes 5. Next, a plunge cutter 1 equipped with a circular saw-shaped blade 7 reaching both insertion holes 5 is inserted in the circular hole 4 and the blade 7 is matched with the cutting scheduled surface 3 and a motor is driven to rotate the blade 7 to push in the same using the circular hole 4 as a guide to cut the cutting scheduled surface 3 and a part of the plugs 6.