PURPOSE: To prevent data with high security from being read by a person except a data owner by providing a filter part which masks the data of a memory.

CONSTITUTION: This device is equipped with a filter 4 which masks the arbitrary bit of the data of a memory 1 by using the data designated by the arbitrary address of the memory 1. This filter part 4 is equipped with an inside data bus 12 from the memory 1, RAM 13 which preserves the data for mask, RAM 14 which preserves the data designated by the arbitrary address of the memory 1, exclusive logical sum arithmetic part 15 arranged for each bit of the data, and outside interface part 16 in which each bit of an output is set as '0' regardless of a signal from the inside data bus 12 at the time of receiving '1' for each bit of the exclusive logical sum arithmetic part 15. Thus, the data of the memory 1 can be prevented from being read by changing data to be written in the RAM 13.