PURPOSE: To keep a charge to be accumulated in the cell of a PROM more than fixed quantity and to enlarge the using environment of the PROM by writing data, which are written in the PROM, again and changing the period of a signal to instruct rewriting.

CONSTITUTION: According to the signal to instruct the rewriting, the data are written again by once writing the data, which are read out of the cell of the PROM (nonvolatile memory), to a register 109 and writing these data from the register 109 to the cell of a PROM 103 again. Accordingly, even at a high temperature or after the lapse of a long period, the data written to the cell of the PROM 103 can be prevented from being erased. Thus, even when the data written to the memory cell are easily erased at the high temperature, the data can be held as they are at the time of writing and limit concerning the using environment can be released.