In a path determination unit of a SAS expander connected to a SAS initiator and connected via first and second paths to a SAS target, an SSP controller receives an SSP command frame received from the SAS initiator; a requested-data-length manager stores a requested data length of the SSP command frame in a requested-data-length storage unit; and a data-transfer-amount manager selects one of the first and second paths having a smaller one of the data transfer amounts stored in a data-transfer-amount storage unit, and adds the requested data length to the data transfer amount of the selected path. The SSP command frame is transmitted to the SAS target via the selected path. Upon receipt of an SSP response frame responding thereto, the requested data length is deleted from the requested-data-length storage unit, and the requested data length is subtracted from the data transfer amount of the selected path.