A unit, which is configured to be attachable to and detachable from a body of an apparatus, includes: a holding portion for holding the unit to attach and detach the unit to and from the apparatus body, the holding portion being projected from a housing of the unit; and a lock mechanism for fixing the unit to the apparatus body, wherein an operating portion of the lock mechanism is operated to retract inside a projection end portion of the holding portion, by an operation of moving the lock mechanism to a lock cancel position.