An inertial rotation device includes: a central member with an axial direction, a first engaging portion on the peripheral surface of the central member, an inertia member with a mounting hole, and a second engaging portion on the wall of the mounting hole. The inertia member is mounted around the central member and can be displaced between an engaging position and a released position along the axial direction of the central member. The inertia member can rotate together with the central member when at the engaging position, where the two engaging portions are engaged with each other, and cannot rotate together with the central member when at the released position, where the two engaging portions are disengaged from each other. The magnitude of the torque of the inertial rotation device can be changed by switching the inertia member to the engaging position or the released position.