Fault tolerant system and controller, access control method, and control program used in the fault tolerant system

In both systems, fault-tolerant system configuration, the router (113), to the IO devices (123) to access the data in the packet, from a given CPU (111) comprising an access source and destination ID code access, and whether it is synchronous access tag information synchronization information. Access comparison unit (133), comprising: from the CPU (111) are held packet buffer (134, 135) in each system; in the packet-based (134, 135) to the buffer conferred flag information, determines whether the packet is synchronous access flag checking unit (136); when the packet is synchronous access, access data packets from one system (121a) outputs IO iF to delete access data from the rest of the system comparing portion (137) of the package. Thus, in the fault-tolerant system, the access processing can be realized corresponding to the synchronous / asynchronous state of the processor.