PURPOSE: To prevent data from being erased by saving write waiting data, which are temporarily stored in a cache memory, in another nonvolatile storage medium when any fault is generated at an optical disk device and writing those data in an optical disk after the fault is recovered.

CONSTITUTION: Concerning the optical disk system for performing the write processing of data to the optical disk loaded on the optical disk device through the cache memory, when any fault is generated at the part of the optical disk device and write error is generated at any one of optical disk driving devices 205 to 208, a device control part 111 for optical disk reports the error to a CPU 101. The CPU 101, that receives this report, saves the write waiting data inside a cache memory 112 into a saving area 402 of a magnetic disk device 110. Afterwards, when the trouble of an optical disk library device 113 is recovered and a normal operation is confirmed, the CPU 101 reads the data saved in the saving area 402 and writes them to the optical disk library device 113 again.