A relay device communicates with first and second terminal devices via first and second communication networks, respectively. The relay device determines whether or not a line stability and a line speed of each of the first and second communication networks satisfy a predetermined condition. When a line stability and a line speed of each of the first and second communication networks satisfy the predetermined condition, the relay device temporarily stores, in a memory, data received from the first terminal device via the first communication network, and transfers the temporarily stored data to the second terminal device via the second communication network.