The present invention provides means for effectively reducing the amount of data by means of de-duplication in a disk array apparatus having a data guarantee code. A control means for the disk array apparatus that adds a data guarantee code to each logical data block and checks the data guarantee code when reading data has a de-duplication performing function and control means for: generating LA substitution information for a function checking the data guarantee code or read data location address substitution information when performing the de-duplication and storing data; performing the de-duplication using the above-mentioned information when reading data; and thereby avoiding false diagnosis of the data guarantee code check.