A gateway device includes plural electronic circuit boards with redundant structures, between first and second networks, which are different. The gateway device receives data provided through the first network and, after applying data processing, outputs data to the second network. Data processing reference timings and time information, which are common in the gateway device, are shared, the data processing is executed in synchronization with the reference timings, and synchronized packets are provided. System switching between the electronic circuit boards is executed in accordance with the time information. Thus, packet outputs are synchronously switched.