A combination of a base attachment structure and a cutting tool, where the base attachment structure includes a connection bolt (8) inserted and movably fitted in an aperture (9a) formed in a curved plate (9) of a gear housing (2) and a cross-shaped slide aperture (11) formed in an arch-shaped projection member (10) extending from a base (7). The connection bolt (8) includes a first upper threaded portion (8a) having threads running in a first direction to form a left-handed screw, and a second lower threaded portion (8b) having threads running in a second direction, opposite to the first direction, to form a right-handed screw. The first threaded portion (8a) is fitted into a hole (12a) formed in a pressure plate (12), whereas the second threaded portion (8b) pierces into an aperture (13a) formed in an arch-shaped clamp plate (13) and is then screwed to a right-handed fixation nut (16). The curved plate (9) of the gear housing (2) and the arch-shaped projection member (10) of the base (7) are interposed between the pressure plate (12) and the clamp plate (13).