PURPOSE: To execute data transfer between main storage devices without increasing hardware quantity and complicating hardware concerning an information processing system having a CPU, I/O processor and main storage device.

CONSTITUTION: When an inter-main memory data transfer request is issued, a main storage device 300 receives this request from a system bus 500 to an address command register 421, and transfer data are passed through an address command buffer 422 and read from a main memory 302. These data are held in a read data register 411 together with a write address from an adder 436 and a write command from a command generating circuit 437, sent to a read data buffer 421 and issued to the system bus 500 as a write request. This write request is received by the same or different main storage device and written in the main memory by a write data register 401 and a write data buffer.