An image forming apparatus comprising: an apparatus main assembly; a unit detachably mountable to the apparatus main assembly; a first coupling (41; 51), provided on the apparatus main assembly, for being rotated by power from a driving source; and a second coupling (42; 52), provided on the unit, for being rotated by engagement with the first coupling (41; 51), wherein the second coupling (42; 52) includes a plurality of projections (42a, 42f; 42g, 42h; 52a, 52f, 52g, 52h) and the first coupling (41; 51) has a T-shaped projection (41b; 51b) and a ring portion surrounding the T-shaped projection (41b; 51b), wherein at least one of an inner surface of the ring portion and an outer surface of at least one of the projections (42a, 42f; 42g, 42h; 52a, 52f, 52g, 52h) includes an inclined surface (41e; 51e; 42e; 52e; 42i), wherein the second coupling (42; 52) is retractable in a direction parallel to a rotational axis thereof, wherein the rotational axis of the second coupling (42; 52) is movable relative to a rotational axis of the first coupling (41; 51) in a disengaging direction of the unit by a force exerted in the disengaging direction of the unit, the disengaging direction being perpendicular to the rotational axis of the second coupling (42; 52), and wherein, when the unit is pulled out from the apparatus main assembly in the disengaging direction, the second coupling (42; 52) starts retracting in the direction parallel to the rotational axis of the second coupling (42; 52) by contact at the inclined surface (41e; 51e; 42e; 52e; 42i) and such relative movement of the rotational axes of the first and second couplings (41, 42; 51, 52). an apparatus main assembly; a unit detachably mountable to the apparatus main assembly; a first coupling (41; 51), provided on the apparatus main assembly, for being rotated by power from a driving source; and a second coupling (42; 52), provided on the unit, for being rotated by engagement with the first coupling (41; 51), wherein the second coupling (42; 52) includes a plurality of projections (42a, 42f; 42g, 42h; 52a, 52f, 52g, 52h) and the first coupling (41; 51) has a T-shaped projection (41b; 51b) and a ring portion surrounding the T-shaped projection (41b; 51b), wherein at least one of an inner surface of the ring portion and an outer surface of at least one of the projections (42a, 42f; 42g, 42h; 52a, 52f, 52g, 52h) includes an inclined surface (41e; 51e; 42e; 52e; 42i), wherein the second coupling (42; 52) is retractable in a direction parallel to a rotational axis thereof, wherein the rotational axis of the second coupling (42; 52) is movable relative to a rotational axis of the first coupling (41; 51) in a disengaging direction of the unit by a force exerted in the disengaging direction of the unit, the disengaging direction being perpendicular to the rotational axis of the second coupling (42; 52), and wherein, when the unit is pulled out from the apparatus main assembly in the disengaging direction, the second coupling (42; 52) starts retracting in the direction parallel to the rotational axis of the second coupling (42; 52) by contact at the inclined surface (41e; 51e; 42e; 52e; 42i) and such relative movement of the rotational axes of the first and second couplings (41, 42; 51, 52).