A PCI-PCI bridge is composed of two physically different controllers, i.e., a primary PCI serial transfer controller arranged at a PC, and a secondary PCI serial transfer controller arranged at a docking station. In data transfer between these controllers, preliminary transmission of block data having a block ID assigned thereto and a return of ACK having the block ID assigned thereto are performed asynchronously. The transmission party can determine which data item is the last the receiving party has received by employing a block ID assigned to NAK. When NAK is received from the receiving party, re-transmission from a block waiting for NAK is started. In this manner, data required to transmit a bus transaction between buses can be transferred accurately and speedily between two controllers.