An image forming apparatus includes a transfer unit detachably supported by a body frame to transfer an image on an image holding member to a recording medium. A driving unit installed on the body frame provides a rotational driving force to the transfer unit. A waste developing agent container is selectively attached to the body frame to store waste developing agent collected from the transfer unit. An engagement assembly is installed on the body frame to selectively engage the transfer unit with the driving unit to the transfer unit while at the same time allowing or blocking the installation of the waste developing agent container in the body frame in accordance with the state of engagement of the transfer unit.