PURPOSE: To clear a cache memory at a time, when the normal execution of a program is not hindered, and before the access of a shared area, by providing a flip flop for storing an error and storing the error of a common line because of which clearing for every entry is impossible.

CONSTITUTION: If a parity error is detected on a zone/command line 16 by a parity checker 31 when a line ZFSTCM 17 is "1" or a parity error is detected on an address/data line 15 by a parity checker 32 when the line 17 is "1" and a command to update contents of a main storage exists, a flip-flop F/F2 33 is set. When firmware checks a gate word to judge that the access to the shared area is permitted, it checks the state of the flip-flop F/F2 33; and if this flip flop F/F2 is set, all entries whose contents do not coincide with contents of the main storage probably are cleared, and the flip flop F/F2 33 is reset.