To provide a power transmission device allowing simple mounting on a follower side apparatus.

A rotor 3 whereto a pulley 4 made by sheet metal processing is supported rotatably by a cylindrical projection A of a compressor. A holding member 5 provided with a plurality of recesses 5a protruding outward in radial direction is fixed to the rotor 3 by rivets. A follower side rotary member 6 provided with a plurality of recesses 6d recessed inward in radial direction is fixed to the rotary shaft B of the compressor by means of spline fitting. The disc part 3c of the rotor 3 and a flange 6c on the member 6 are positioned opposing in the axial direction, and a coupling member 9 formed cylindrically from plastics or rubber is fitted in the space bounded by the recesses 5a and 6d, disc part 3c, and flange 6c. When the rotary shaft B of the compressor is locked, the coupling member 9 is broken and the power transmitted is shut off.