PURPOSE: To realize reduction of the overhead and the flexible control for an input/output device having the optional variable data record length, by restarting the transfer of data between a peripheral controller and a main memory for each prescribed byte unit based on the remaining data length and a restart address.

CONSTITUTION: A restart address of the data to be transferred is calculated based on the remaining data length that is calculated from the length of the data to be transferred among peripheral controllers 3-1W3-n as well as the length of the data processed by its own device. Then the transfer of data is restarted for each prescribed byte until between the controllers 3-1W3-n and a main storage device 1 based on said remaining data length and restart address. Thus it is possible to reduce the overhead and to perform the flexible control even with an input/output processor 2 having its variable data record length even though the data transferred is not equal to an integer multiple of the data transfer unit.