PURPOSE: To prevent an input/output device from outputting erroneous data by switching a controller which becomes an operating state.

CONSTITUTION: In the case the controller being in the course of operation finishes the execution (Y) of a user program and causes system down in the course of executing (Z) a new user program, and the controller being in the course of holding is switched to an operating state before continuous data outputted in the end of the execution (y) of the user program is received by the standby side controller, its controller which becomes newly an operating state executes an access for inquiring function information of an input/output device 300, based on device information being a part of the continuous data, in the course of execution (Y) of the user program executed in the beginning. In response to its inquiry, the input/output device 300 transfers the function information to the new operating controller, and also, outputs continuously transfer data YO.