An image forming apparatus comprising: a main assembly; a driving source provided in said main assembly; a first coupling provided in said main assembly and rotatable by a driving force from said driving source, said first coupling is capable of transmitting the driving force by an engaging portion (51, 51a) provided at an end portion thereof in a first direction (A2, A4) which is a direction of a rotational axis of said first coupling and being movable in the first direction (A2, A4), said first coupling being provided at a side surface of said engaging portion (51, 51a) with a coupling taper surface having a diameter which increases in the first direction (A2, A4) from a free end portion toward said driving source; and a unit detachably mountable to said main assembly in a second direction (A1, A3) crossing with the first direction (A2, A4), said unit including a second coupling, a driven member and a releasing member, wherein said second coupling is rotatable and is disposed such that when said unit is mounted to said main assembly in a mounted position, a rotational axis of said second coupling and the rotational axis of said first coupling are substantially coaxial with each other, and said second coupling includes at an end portion thereof with respect to a direction of the rotational axis of said second coupling, a portion-to-be-engaged engageable with said engaging portion (51, 51a) to transmit the driving force from said driving source, said driven member is configured to receive the driving force from said second coupling, and wherein said releasing member is movable relative to said second coupling in a direction crossing with the direction of the rotational axis of said second coupling, and wherein when said unit is in the mounted position, said releasing member in contact with said coupling taper surface moves in the second direction (A1, A3) to move said first coupling away from said second coupling to effect disengagement between said engaging portion (51, 51a) and said portion-to-be-engaged. a main assembly; a driving source provided in said main assembly; a first coupling provided in said main assembly and rotatable by a driving force from said driving source, said first coupling is capable of transmitting the driving force by an engaging portion (51, 51a) provided at an end portion thereof in a first direction (A2, A4) which is a direction of a rotational axis of said first coupling and being movable in the first direction (A2, A4), said first coupling being provided at a side surface of said engaging portion (51, 51a) with a coupling taper surface having a diameter which increases in the first direction (A2, A4) from a free end portion toward said driving source; and a unit detachably mountable to said main assembly in a second direction (A1, A3) crossing with the first direction (A2, A4), said unit including a second coupling, a driven member and a releasing member, wherein said second coupling is rotatable and is disposed such that when said unit is mounted to said main assembly in a mounted position, a rotational axis of said second coupling and the rotational axis of said first coupling are substantially coaxial with each other, and said second coupling includes at an end portion thereof with respect to a direction of the rotational axis of said second coupling, a portion-to-be-engaged engageable with said engaging portion (51, 51a) to transmit the driving force from said driving source, said driven member is configured to receive the driving force from said second coupling, and wherein said releasing member is movable relative to said second coupling in a direction crossing with the direction of the rotational axis of said second coupling, and wherein when said unit is in the mounted position, said releasing member in contact with said coupling taper surface moves in the second direction (A1, A3) to move said first coupling away from said second coupling to effect disengagement between said engaging portion (51, 51a) and said portion-to-be-engaged.