To maintain reliability even when the repetition of the power saving control for storage systems occurs frequently due to the access from the host computer. During the course of controlling the operation mode of the drives, the controller measures the start/stop cycle count and the load/unload cycle count, calculates the S/S wear-out rate showing the ratio of the measured value of the start/stop cycle count to the first upper limit value and the L/U wear-out rate showing the ratio of the measured value of the load/unload cycle count to the second upper limit value, and for the drives in the idle operation mode, selects the low-speed rotation operation mode if the S/S wear-out rate is greater than the L/U wear-out rate, or selects the standby operation mode if the S/S wear-out rate is less than the L/U wear-out rate.