PURPOSE: To realize a data protective function at high reliability by reserving the contents of a RAM to an E2PROM when a built-in battery shifts to a power source state in which the normal storing operation of the RAM cannot be maintained.

CONSTITUTION: A low battery detecting circuit 7, which detects the power source decreasing state of a built-in battery 6, and a control circuit 8, which receives a detecting signal indicating the power source decreasing state and reserves the data stored into a RAM 2 to an E2PROM 4, are provided. When the built-in supply voltage drops to a critical point to maintain the normal storage operation of the RAM 2, the contents of the RAM 2 are reserved to the E2PROM 4. Thus the contents breakdown of the RAM data corresponding to the decrease of the built-in battery 6 is evaded, the highly reliable data protective function can be realized, and by using the function for the external memory medium of a personal computer, the memory card can be easily made smaller and lighter.