PURPOSE: To reduce the hardware without increasing the load and processing time of a CPU by using only one mask circuit system.

CONSTITUTION: Data are stored in plural memories M1 to Mn without masking them. When one memory M is selected out of the plural memoris M1 to Mn by a bus interface 4, the CPU 1 rewrites the contents of a register R to masking data corresponding to the selected memory M and the masking data are supplied to a masking circuit S. The circuit S masks bits other than objective bits out of data stored in the selected memory M based upon the masking data and only specific bits to be processed are outputted to the CPU 1.