A rotation transmission device comprising: an outer ring; an inner member arranged inside of the outer ring, and supported so as to be rotatable relative to the outer ring; engaging elements (3a and 3b) provided between an inner periphery of the outer ring and an outer periphery of the inner member; an engaging element retainer supported so as to be movable between an engagement position in which the engaging elements (3a and 3b) are engaged with the outer ring and the inner member such that rotation is transmitted between the outer ring and the inner member through the engaging elements (3a and 3b), and a disengagement position in which the engaging elements (3a and 3b) are disengaged from the outer ring and the inner member such that the rotation is not transmitted between the outer ring and the inner member; an armature supported so as to be axially movable; a rotor supported so as to be axially unmovable, and axially opposed to the armature; a spring member biasing the armature away from the rotor; an electromagnet configured to attract the armature to the rotor when the electromagnet is energized; and a motion converting mechanism configured to convert a motion of the armature when attracted to the rotor into a movement of the engaging element retainer from one of the engagement position and the disengagement position to the other of the engagement position and the disengagement position, wherein the rotation transmission device further comprises a buffer provided between the armature and the rotor so as to absorb a shock generated when the armature is attracted to the rotor, characterized in that the buffer comprises a metal annulus axially movably supported by one of the armature and the rotor, and a rubber annulus provided between the metal annulus and said one of the armature and the rotor such that as the armature approaches the rotor, the rubber annulus is axially compressed by the metal annulus and said one of the armature and the rotor, and wherein the rubber annulus has different axial thicknesses at different circumferential locations of the rubber annulus such that when a degree of axial compression of the rubber annulus is large, the rubber annulus is axially compressed in a longer circumferential area of the rubber annulus than when the degree of axial compression of the rubber annulus is small. an outer ring; an inner member arranged inside of the outer ring, and supported so as to be rotatable relative to the outer ring; engaging elements (3a and 3b) provided between an inner periphery of the outer ring and an outer periphery of the inner member; an engaging element retainer supported so as to be movable between an engagement position in which the engaging elements (3a and 3b) are engaged with the outer ring and the inner member such that rotation is transmitted between the outer ring and the inner member through the engaging elements (3a and 3b), and a disengagement position in which the engaging elements (3a and 3b) are disengaged from the outer ring and the inner member such that the rotation is not transmitted between the outer ring and the inner member; an armature supported so as to be axially movable; a rotor supported so as to be axially unmovable, and axially opposed to the armature; a spring member biasing the armature away from the rotor; an electromagnet configured to attract the armature to the rotor when the electromagnet is energized; and a motion converting mechanism configured to convert a motion of the armature when attracted to the rotor into a movement of the engaging element retainer from one of the engagement position and the disengagement position to the other of the engagement position and the disengagement position, wherein the rotation transmission device further comprises a buffer provided between the armature and the rotor so as to absorb a shock generated when the armature is attracted to the rotor, characterized in that the buffer comprises a metal annulus axially movably supported by one of the armature and the rotor, and a rubber annulus provided between the metal annulus and said one of the armature and the rotor such that as the armature approaches the rotor, the rubber annulus is axially compressed by the metal annulus and said one of the armature and the rotor, and wherein the rubber annulus has different axial thicknesses at different circumferential locations of the rubber annulus such that when a degree of axial compression of the rubber annulus is large, the rubber annulus is axially compressed in a longer circumferential area of the rubber annulus than when the degree of axial compression of the rubber annulus is small.