The reliability of output data is enhanced, and the frequency of stopping arithmetic devices is reduced. A redundant system includes an input device; a plurality of arithmetic devices that receive input data from the input device; and an output device that receives output data output from the arithmetic devices, the redundant system causing the arithmetic devices to perform the same processing. Each of the arithmetic devices includes: a first communication unit that acquires the input data from the input device; and a second communication unit that sends the input data acquired by the first communication unit to other arithmetic device and receives the input data acquired by the other arithmetic device from the other arithmetic device.