PURPOSE: To transmit large power without enlarging the whole device.

CONSTITUTION: A cylindrical torque transmission boss section 30a formed with a spline 32a on the inner periphery is fixed to the boss section 30a of a coupling 30 coupled with a coupling on the output shaft side of an engine, a sleeve 34 rotatably supported by a bearing 36 is arranged coaxially with the torque transmission member 31, and the end section 34a of the sleeve 34 is fixed to the end section of the torque transmission member 31. A propeller shaft 15 formed with a spline 15a on the outer periphery of the tip section is inserted into the sleeve 34, and the propeller shaft 15 is spline-coupled with the torque transmission member 31.