PURPOSE: To provide a system for writing data in a disk array. CONSTITUTION: This system includes a cache memory 31 connected with disk arrays 40-43, which stores data indicating positions on the disk arrays and party blocks related with a parity group including the positions. Each parity block includes an identifier indicating the position in the specific parity group protected by parity data. A writing logical mechanism reads the identifier from the parity block, and decides whether or not the data position is protected by the parity data based on the identifier. Also, for including the position of the data block for indicating that the position is protected, the writing logical mechanism writes the parity data and the identifier related with the parity block in the position, and updates it. COPYRIGHT: (C)1995,JPO