PURPOSE: To correct a memory error with application of a parity check system by usually performing the horizontal parity check to the memory read data and then carrying out the vertical parity check only when a parity error is detected.

CONSTITUTION: A memory 11 is logically divided into blocks BL0 - BLN-1 in the address direction, and a parity bit is added to each block in both horizontal and vertical directions. In a read access state of the memory 11, the horizontal parity check is carried out. If a parity error is detected in the horizontal parity check, the vertical parity check is carried out to the corresponding block. If a parity error is detected in the vertical parity check, the bit position shows a position of an error bit in the word data received a memory read access. Thus the error is easily corrected on the basis of the parity check.