PURPOSE: To improve work efficiency for patching a control program stored in the ROM of an input/output device.

CONSTITUTION: An external storage device 2 preserves patch data 21 of the control program of an input/output device 3. A patch data reading means 11 of an information processing system 1 reads the patch data from the external storage device 2 and stores them in a patch data storage part 12. A patch data transfer means 13 transfers the patch data inside the patch data storage part 12. A ROM 31 of the input/output device 3 stores the control program. A program allocating means 33 allocates the control program inside the ROM to a memory part 32 and starts a system by operating the control program. A data receiving means 34 receives the patch data transferred from the patch data transfer means 13 after the system is started. A data preservation part 35 temporarily preserves the received patch data. A patch means 36 patches the patch data in the data preservation part to the control program on the memory part.