A RAID group of RAID 1 series comprises one or more pairs of first storage devices and second storage devices. A storage apparatus reads data from the entire area of a first storage block group including the write destination of write target data in the first storage device. The storage apparatus, in accordance with the write target data and staging data which is the read data, generates one or more data units each of which is the data configured of the write target data or the copy of the same and the staging data part or the copy of the same and of the same size as the first storage block group. The controller writes any of the one or more data units to the first storage block group in the first storage device and, at the same time, writes any of the one or more data units to the second storage block group corresponding to the first storage block group and of the same size as the same in the second storage device.