According to an aspect of an embodiment, a method for controlling a controller connected to a plurality of storage units storing data, the controller including a cache and a buffer, the method comprising the steps of: storing data in the cache; generating parity data corresponding to the data and storing the parity data in the buffer; writing the data and the parity data into the plurality of the storage units; comparing the parity data stored in the buffer with the parity data written into and read out from at least one of the storage unit; deleting, when the parity data stored in the buffer is different from the parity data read out from the storage unit, the parity data from the buffer; and regenerating parity data from data stored in the cache and rewriting the regenerated parity data into at least one of the storage unit.