STORAGE DEVICE

PURPOSE:To provide a memory card which is long in the life and is high in reliability. CONSTITUTION:A memory card 100 is composed as a buffer storage device where a flash memory is used as a main storage device and a static RAM is used for a cache memory 2. A copy/back system is adopted as the data siting system to the main storage device 1, a battery backuping the static RAM as the cache memory 2 and a power source voltage detection switch circuit 4 is incorporated in the card, the power source is switched to the battery power source when external power source voltage is interrupted, and the battery power source backups the cache memory 2. Therefore, a normal write operation is performed for only the static RAM as the cache memory 2. Because a writing is performed for the flash memory 1 as the main storage device 1 only when the data block within the cache memory 2 for which the writing is performed is replaced, the number of times of writing in the flash memory is reduced, and the reliability are improved.