A fault tolerant (FT) computer system includes a first system; and a second system configured to operate in synchronization with the first system. Each of the first and second systems includes a CPU; and a routing controller connected with the CPU. The first system includes a first I/O device as an activist I/O device, and the second system includes a second I/O device as a standby I/O device. The routing controller controls a routing between the CPU and the first I/O device and the second I/O device. When a fault has occurred in the first I/O device, the routing controller in said first system routes a request data a request data received from the CPU and destined to the first I/O device, to the second I/O device.