Accordingly, a difference between the remaining life 20b1 of the added SSD 20b and the remaining life 20a1 of the existing SSDs 20a decreases with time. The storage controller 10 carries out the second-order distribution by carrying out the second data distribution processing in which the number of pieces of the user data configuring the parity group is increased at the time T2 when the life difference resulting from subtraction of the remaining life 20a1 of the existing SSDs 20a from the remaining life 20b1 of the added SSD 20b is obliterated and thereby changes the RAID configuration into 3D+1P. Even though the life difference is not obliterated, it is sufficient if timing of start of the second data distribution processing is determined based on the life difference. For instance, the storage controller 10 may carry out the second-order distribution on condition that the life difference reaches a threshold. In the second data distribution processing, it is sufficient if the number of pieces of the user data is increased from a first number to a second number greater than the first number. Then, a data volume of the redundant data with respect to the user data is decreased.