PURPOSE: To prevent previously the crush of a head by attaching a metallic oxidation detector to a magnetic disk to detect previously the oxidation of a metallic base material of the magnetic disk.

CONSTITUTION: A magnetic disk is provided with a motor 2, control circuit 3 which control the motor 2, metallic oxidation sensor 41 containing a metal printed on a substrate, and a metallic oxidation detector 4 consisting of a detecting circuit 42 which detects the change of an oxidation signal. A metallic thin film is gradually oxidized by the water or gas contained in the air, and therefore the resistance value of the sensor 41 is also increased gradually. The circuit 42 monitors the resistance value of the sensor 41. This resistance value is detected when it exceeds a prescribed level. Then a fault signal is transmitted to the circuit 3.