PURPOSE: To enable a main shaft to be fixed tightly even if a torsion coil spring is not strong by constituting a part where the torsion coil spring in a bent section-fixing hole is caught in such a way that a distance from the center of rotation may increase in a direction in which the bent section for fixing returns.

CONSTITUTION: A torsion coil spring 5 has a bent section for fixing 5b inserted in a bent section-fixing hole 4b in a chassis 4 in such a condition that the spring 5 has both arms contracted under pressure. The torsion coil spring 5 is caught into the chassis 4 by extending spring force 7. Tension 8 generates in the arm of the torsion coil spring 5 in such a manner as to balanced against the spring force 7. And a reaction 9 generates against the force by which the bent section 5b of the torsion coil spring 5 is held into the bent section-fixing hole 4b of the chassis 4. The reaction 9, coupled with a reaction by another bent section for fixing 5c of the torsion coil spring 5, causes a coil 5a to press a main shaft 2 to a V-shaped part of a main shaft fixing hole 4a in the chassis 4, thus fixing the main shaft.