An information processing system that transfers transactions between a plurality of system modules. A request side interface unit in a request side module has a request ID queue in which issued request transactions are stored in order of issuance. A request side queue pointer points to an entry in this request ID queue corresponding to a response transaction to be accepted next. A response side interface unit in a response side module has a response queue in which accepted request transactions are stored in order of acceptance. A response side queue pointer points to an entry in this response queue corresponding to a response transaction to be issued next. Therefore, a request transaction and the corresponding response transaction are transferred between the request side interface unit and the response side interface unit without transferring transaction IDs. When the response order is changed, the response side interface unit issues a command, which changes the value of the request side queue pointer, to inform the request side interface unit of the change in the order.