PURPOSE: To attain one-bit writing by reading out one data, padding bit data in a bit position estimated by a decoder and writing the one data in a RAM.

CONSTITUTION: The writing and reading access of plural bits of data in/from a RAM is simultaneously executed. Thereby the reading of data from the outside of a buffer 11 and from the RAM 10 is executed and in this case, only a bit to be written is read out from the outside, data other than the bit to be written are read out from the RAM and the read data are stored in the buffer 11. Only the data of one bit are written by writing the data stored in the buffer 11 in the RAM 10. Namely writing one-bit data are padded in the corresponding bit position of one data specified by a decoder 14 and the one data obtained by padding are read out as parallel bits and written in the original RAM position.