PURPOSE: To continuously use a same portable medium by rewriting a usable amount when the total of the used amount of portable medium arrives at the usable amount set first.

CONSTITUTION: The IC chip of an IC card as the portable medium is constituted of a CPU3 for control and EZPROM and composed of a data memory 4 in which a program for control and data are stored, a memory RAM15 for arithmetic processing, an interface circuit 6 and a connector part 7. In the data memory 4, there are an area (b) in which a first usable amount is stored, a used amount memory area (c) and an updated usable amount memory area (d), a usable balance is minimized, and when a user pays cash for updating, the amount for updating is added to the difference between the usable amount and the used amount and written into the area (d).