PURPOSE: To allow an information storage device to protect general maintenance information storage area by itself during maintenance by providing a switch circuit which puts a maintenance information storage area and a general information storage area in a writable state selectively.

CONSTITUTION: An AND circuit 4 inputs a write instruction from a signal line (8), and when the switch circuit 1 is connected to a signal line (1), only a write instruction when a maintenance cylinder is indicated is stored through an OR circuit 3 with the output of a maintenance cylinder detecting circuit 2 to be outputted to a signal line (9). When the switch circuit 1 is so set that only the maintenance cylinder is placed in a writable state, an AND circuit 5 outputs an error signal to a signal line (10) on obtaining the write instruction from the signal line (8) unless the maintenance cylinder is selected by the maintenance cylinder detecting circuit 2.