A hybrid driving force transmission device is provided with a multiple-plate dry clutch, a piston, a piston arm, a return spring, bellows elastic seal components and an arm fixing plate. The return spring applies biasing force to the piston arm in the release direction of the clutch, during the release period of the multiple-plate dry clutch. The bellows elastic seal member is secured or maintained in a position on the side wall of a clutch drum and seals a through hole and arm distal end from the clutch chamber, and undergoes an elastic deformation following the stroke movement of the arm distal end. The arm fixing plate is integrally provided with the bellows elastic seal member, is fixed to the arm distal end, and when the multiple-plate dry clutch is engaged, is configured to contact the clutch plate for transferring the clutch engagement force.