PURPOSE: To prevent a maintenance attendant from causing the damage to user data when he investigates a trouble by outputting a write permit signal while it is detected that a magnetic head for a disk-like recording medium is positioned to a maintenance cylinder.

CONSTITUTION: If a protect switch (SW) 1 is set to a signal (1) side when a maintenance attendant wants to execute a write instruction in the course of trouble investigation of an information storage device, the signal (1) which sets a maintenance cylinder only to a write enable condition is outputted to a NAND circuit 7 through a NOT circuit 4. When the address of the maintenance cylinder is decoded by a decoder 3 from a cylinder address register circuit 2, the signal is inputted into the circuit 7 through another NOT circuit 5. The output of the circuit 7 is inputted into an AND circuit 8, and the circuit 8 outputs a write instruction (5) when three conditions; a condition that an all cylinder write inhibit instruction signal (2) is not outputted, condition that an inputted write signal (3) catches the maintenance cylinder, and condition that a magnetic head catches the maintenance cylinder, agree with each other. When the SW1 is set to the signal (2) side, the write instruction 3 is outputted from an AND circuit 9 as an error detect signal (5).