According to a prior art data transfer method of a storage subsystem, when competition of data transfer accesses occurs, a free access destination port is allocated uniformly without determining the access type or the access state of the access destination, so that the performance of the device is not enhanced. The present invention solves the problem by selecting a data transfer access for completing data transfer with priority based on the access type or the remaining transfer data quantity of competing data transfer accesses, or by changing the access destination of an access standby data transfer access, thereby performing data transfer efficiently.