To prevent a tightening part from getting loose while solving problems of circle bolt type in the fitting structure of a modular clutch.

In the fitting structure 8 of a modular clutch, the clutch can be assembled as a module in the crank shaft of an engine. Flywheel (4, 5), a clutch cover assembly 6, and a clutch disc assembly 7 are included in the structure. The structure is a device for attaching the modular clutch device 1 to transmit a torque to the input shaft 3 of the transmission to the crank shaft 2. In the fitting structure 8, although the flywheel (4, 5) can not be axially attached or detached to/from the inner periphery of the flywheel (4, 5), they are engaged to be relatively rotatable, and by fastening it to the crank shaft 2, a lockbolt 28 to fix axially inseparably the flywheels (4, 5) to the crank shaft 2 is provided. The lock bolt 28 is a circular member fastened to the outer peripheral face of the crank shaft 2.