The present invention relates to a gateway device, a network system and a data converting method using a plurality of network protocols different from each other. A gateway device includes a first communication controller connected to first device via a first bus in accordance with a first network protocol to exchange data with the first device in accordance with the first network protocol, a second communication controller connected to second device via a second bus in accordance with a second network protocol different from the first network protocol to exchange the data with the second device in accordance with the second network protocol, and a control unit interconverting the data in accordance with the first network protocol and the data in accordance with the second network protocol into each other so that the data is exchanged between the first device and the second device.