A key setting method executed by a node transmitting and receiving data through multi-hop communication in an ad-hoc network among multiple ad-hoc networks, includes detecting connection with a mobile terminal communicating with a server connected to a gateway in each ad-hoc network among the ad-hoc networks; transmitting by simultaneously reporting to the ad-hoc network, an acquisition request for a key for encrypting the data when the connection with the mobile terminal is detected at the detecting; receiving from the server via the mobile terminal, a key specific to a gateway and transmitted from the gateway to the server consequent to transfer of the simultaneously reported acquisition request to the gateway in the ad-hoc network; and setting the key specific to the gateway received at the receiving as the key for encrypting the data.