An image forming apparatus comprising: an apparatus main assembly; a unit detachably mountable to said apparatus main assembly; a first coupling (41; 51; 61), provided on said apparatus main assembly, for being rotated by power from a driving source; and a second coupling (42; 52; 62), provided on said unit, for being rotated by engaging with said first coupling (41; 51; 61), wherein said first coupling (41; 51; 61) has a recessed shape (41b; 51b) and said second coupling (42; 52; 62) has a projected shape (42a; 52a), wherein said first coupling (41; 51; 61) and/or said second coupling (42; 52; 62) have/has an inclined surface (41e; 51e; 61e; 42e; 52e; 62e), wherein said first coupling (41; 51; 61) and/or said second coupling (42; 52; 62) are/is retractable toward a direction parallel to a rotation axis thereof, and wherein said image forming apparatus has a structure such that a rotation axis of said second coupling (42; 52; 62) is movable relative to a rotation axis of said first coupling (41; 51; 61) in a disengaging direction of said unit by a force exerted in the disengaging direction of said unit when said unit is pulled out from said apparatus main assembly in said disengaging direction perpendicular to the rotation axis of said first coupling (41; 51; 61) and then by contact of the inclined surface (41e; 51e; 61e; 42e; 52e; 62e) with the other coupling and by such relative movement of the rotation axes, said first coupling (41; 51; 61) and/or said second coupling (42; 52; 62) are/is retractable along the inclined surface (41e; 51e; 61e; 42e; 52e; 62e) in the direction parallel to the rotation axis from a state in which said first and second couplings (41; 42; 51; 52; 61; 62) are engaged to be rotatable together to a state in which said first and second couplings are disengaged. an apparatus main assembly; a unit detachably mountable to said apparatus main assembly; a first coupling (41; 51; 61), provided on said apparatus main assembly, for being rotated by power from a driving source; and a second coupling (42; 52; 62), provided on said unit, for being rotated by engaging with said first coupling (41; 51; 61), wherein said first coupling (41; 51; 61) has a recessed shape (41b; 51b) and said second coupling (42; 52; 62) has a projected shape (42a; 52a), wherein said first coupling (41; 51; 61) and/or said second coupling (42; 52; 62) have/has an inclined surface (41e; 51e; 61e; 42e; 52e; 62e), wherein said first coupling (41; 51; 61) and/or said second coupling (42; 52; 62) are/is retractable toward a direction parallel to a rotation axis thereof, and wherein said image forming apparatus has a structure such that a rotation axis of said second coupling (42; 52; 62) is movable relative to a rotation axis of said first coupling (41; 51; 61) in a disengaging direction of said unit by a force exerted in the disengaging direction of said unit when said unit is pulled out from said apparatus main assembly in said disengaging direction perpendicular to the rotation axis of said first coupling (41; 51; 61) and then by contact of the inclined surface (41e; 51e; 61e; 42e; 52e; 62e) with the other coupling and by such relative movement of the rotation axes, said first coupling (41; 51; 61) and/or said second coupling (42; 52; 62) are/is retractable along the inclined surface (41e; 51e; 61e; 42e; 52e; 62e) in the direction parallel to the rotation axis from a state in which said first and second couplings (41; 42; 51; 52; 61; 62) are engaged to be rotatable together to a state in which said first and second couplings are disengaged.