PURPOSE: To easily perform a changing process at changing a computer system.

CONSTITUTION: A device definition information file F constituted of protocol information, driver information, and the like necessary for data access to respective control devices is previously prepared in a device information preparation part 15 and stored in a device information storing part 16, and when a data access program is carried out, a protocol processing part 22 refers to the device definition information file F stored in the device information storing part 16, starts a decided driver drive part indicated by the device definition information file F, has access to an interface card, and hence carries out data access to a decided control device, and when a computer system is changed, by changing the decided parameter in the device definition information file F by means of the device information preparation part 15, the computer system can be changed without changing the data access program of the computer.