A first transmitting unit transmits a processing command to a plurality of parallelized database servers. A second transmitting unit integrates data sets transmitted from the database servers in response to the processing command, and transmits an integrated data set to a client. An integrating unit integrates data sets buffered in a buffer unit. A determining unit determines a transmission start or a transmission suspend of the data sets based on a data size in the buffer unit. A third transmitting unit transmits a control command for the transmission start or the transmission suspend to the database servers based on a result of determination by the first determining unit.