A control system, which is capable of performing a fault-tolerant function realized in the manner of duplication by merely transferring memory ownership from one processor module to another with the use of a method of extending a memory bus, is provided. Each processor module in the control system includes an A-port memory and a B-port memory which store memory data of their own processor module (the self processor module) or memory data of the other processor module and a memory bus switch which performs switching-over to selectively store data of the self processor module or data transmitted from the other processor module in the A-port or B-port memory, depending on whether the self processor module or the other processor module has memory ownership.