If the data distribution state 533 indicates “FIRST ORDER,” that is, if the parity group PG is in the first-order distribution state, the storage controller 10 calculates the life difference among the SSDs 20 included in the parity group PG on the basis of the life statistics table 550 and determines whether the life difference is equal to or greater than the threshold or not (step S202). Specifically, the life difference is a value resulting from subtraction of a value based on the remaining lives 554 of the existing SSDs 20a from the remaining life 554 of the added SSD 20b. The value based on the remaining lives 554 may be a statistic of the remaining lives 554 of the existing SSDs 20a or may be the remaining life 554 of an SSDa selected from among the existing SSDs 20a by a specified method. The statistic is an average value, maximum value, minimum value, or the like, for instance. The threshold is approximately between 20% and 30%, for instance.