A smartcard includes a write unit that writes data to be written to a data memory according to a write command supplied from an external device, a determining unit that determines whether important data is contained in the data to be written specified by the write command supplied from the external device, a calculating unit that calculates a head address of the important data in the data memory based on data length from a head of the data to be written to a head of the important data and an address on the data memory at which a process of writing the data to be written is started when the determining unit determines that important data is contained in the data to be written, and a position table that stores information indicating a head address of the important data in correspondence to information indicating the important data.