One or more switches are interposed between a controller portion and a storage device. When transmission of a command to a certain storage device fails, a command is transmitted starting from an upstream side to a downstream side of a path between the controller portion and the switch to which the certain storage device is connected, and when command transmission fails while transmitting a command from a kth switch (k is an integer of 0 or more) which is connected to a (k+1)th switch and is one level upstream of the (k+1)th switch or from any port of the controller portion, it is determined that a failure has occurred in a power source that supplies power to the (k+1)th switch.