PURPOSE: To detect abnormality in mounting of a detachable unit without complicating the construction by applying a signal identical to an abnormality signal with an abnormality detection sensor in the unit to a control section when the unit is poorly connected to the body of a copying machine.

CONSTITUTION: When a connector 12 fails to be connected in mounting a detachable unit to the body of a copying machine, a high level output to be pulled up with a resistance R1 is appied to NAND circuits NAND1 and NAND2 of a control circuit of the body thereof. Therefore, a signal identical to a detection signal as provided when a paper detector in the unit detects jamming or the like to open a switch 11 therein is applied to the control circuit to set an RS type FF15 of the control circuit and a high level Q output as abnormality signal is generated to disable copying. This eliminates any sensor for checking each unit for mounting thereby enabling the detection of abnormality in mounting of units with a simple construction.