An image forming apparatus, comprising: a driving component operable to rotate a driving assembling member about a rotational driving axial line; a developing unit detachably provided in the image forming apparatus that has a driven assembling member capable of being engaged with and disengaged from the driving assembling member in a direction of the rotational driving axial line; a power transmission unit operable to selectively engage a mechanical power to the developing unit; and a manipulating unit operable to manipulate the power transmission unit, wherein at least one of the driving assembling member and the driven assembling member is rotatable in a forward direction about the rotational driving axial line and a reverse direction about the rotational driving axial line, and wherein the power transmission unit is disposed between the driving assembling member and the driven assembling member and comprises a transmission member rotatable in the forward direction and the reverse direction and moveable in the direction of the rotational driving axial line to enable at least one of the driving assembling member and the driven assembling member to move in the direction of the rotational driving axial line upon receipt of a rotational force so as to engage and disengage the driving assembling member with the driven assembling member, and the driven assembling member is operable to be rotated about the rotational driving axial line by the driving component when the driven assembling member and the driving assembling member are engaged; wherein the manipulating unit is operable to engage with the power transmission unit to engage the driving assembling member and the driven assembling member, the manipulating unit comprising an actuator which is operable to engage with the power transmission unit to apply the rotational force to the power transmission unit so as to engage the driving assembling member and the driven assembling member, characterised in that the manipulating unit is operable to disengage from the power transmission unit and the power transmission unit comprises a biasing device configured to cause the transmission member to rotate in the reverse direction when the manipulating unit disengages from the power transmission unit, thereby disengaging the driving assembling member and the driven assembling member. a driving component operable to rotate a driving assembling member about a rotational driving axial line; a developing unit detachably provided in the image forming apparatus that has a driven assembling member capable of being engaged with and disengaged from the driving assembling member in a direction of the rotational driving axial line; a power transmission unit operable to selectively engage a mechanical power to the developing unit; and a manipulating unit operable to manipulate the power transmission unit, wherein at least one of the driving assembling member and the driven assembling member is rotatable in a forward direction about the rotational driving axial line and a reverse direction about the rotational driving axial line, and wherein the power transmission unit is disposed between the driving assembling member and the driven assembling member and comprises a transmission member rotatable in the forward direction and the reverse direction and moveable in the direction of the rotational driving axial line to enable at least one of the driving assembling member and the driven assembling member to move in the direction of the rotational driving axial line upon receipt of a rotational force so as to engage and disengage the driving assembling member with the driven assembling member, and the driven assembling member is operable to be rotated about the rotational driving axial line by the driving component when the driven assembling member and the driving assembling member are engaged; wherein the manipulating unit is operable to engage with the power transmission unit to engage the driving assembling member and the driven assembling member, the manipulating unit comprising an actuator which is operable to engage with the power transmission unit to apply the rotational force to the power transmission unit so as to engage the driving assembling member and the driven assembling member, characterised in that the manipulating unit is operable to disengage from the power transmission unit and the power transmission unit comprises a biasing device configured to cause the transmission member to rotate in the reverse direction when the manipulating unit disengages from the power transmission unit, thereby disengaging the driving assembling member and the driven assembling member.