A first storage virtualization device operates in a system in which instructions requested by host computers are assigned to a plurality of storage virtualization devices. The first storage virtualization device includes an interface and a processor. The interface receives a write instruction assigned to the first storage virtualization device, data to be written by the write instruction, and operation statuses of other storage virtualization devices in the system. The processor calculates index values which represent loads when the first storage virtualization device processes the write instruction. The processor selects a second storage virtualization device for the purpose of requesting that it execute the write instruction by use of the operation statuses when the index values are greater than thresholds. The processor transmits the write instruction and the data to the second storage virtualization device.