1. A communication terminal comprising: a communication unit that is configured to wirelessly transmit and receive communication messages to and from other communication terminals; a data storage unit that is configured to store one or more pieces of data; a specified terminal determination unit that is configured to determine whether or not the own communication terminal is a specified terminal that is authorised to transmit a holding list; a holding list transmission unit that is configured to transmit, only in case the own communication terminal is the specified terminal, the holding list that lists information about the data stored in the data storage unit to the other communication terminals through use of the communication messages; and a data exchange unit that is configured to receive another holding list from one of the other communication terminals, and transmit and receive the communication messages to and from the one of the other communication terminals, wherein the another holding list does not match the holding list of the own communication terminal, and wherein, through the communication messages and based on differences between the holding list and the another holding list, the own communication terminal and the one of the other communications terminals share data not stored in their data storage units, wherein the specified terminal determination unit is configured to determine whether or not the own communication terminal is the specified terminal in accordance with whether or not the own communication terminal functions as a relay terminal, wherein the relay function of the own communication terminal is enabled in response to determining through negotiation between the communication terminals that the own communication terminal is a parent node in a communication network to which the communication terminals are connected, wherein the communication network is a network that has a hierarchical network topology, in which only one of the communication terminals functions as a parent communication terminal and the remaining communication terminals function as child communication terminals, in which the transmission and reception of communication messages between child communication terminals are performed via the parent communication terminal, and in which communication terminals that function as the relay terminals are limited to a subset of all communication terminals. a communication unit that is configured to wirelessly transmit and receive communication messages to and from other communication terminals; a data storage unit that is configured to store one or more pieces of data; a specified terminal determination unit that is configured to determine whether or not the own communication terminal is a specified terminal that is authorised to transmit a holding list; a holding list transmission unit that is configured to transmit, only in case the own communication terminal is the specified terminal, the holding list that lists information about the data stored in the data storage unit to the other communication terminals through use of the communication messages; and a data exchange unit that is configured to receive another holding list from one of the other communication terminals, and transmit and receive the communication messages to and from the one of the other communication terminals, wherein the another holding list does not match the holding list of the own communication terminal, and wherein, through the communication messages and based on differences between the holding list and the another holding list, the own communication terminal and the one of the other communications terminals share data not stored in their data storage units, wherein the specified terminal determination unit is configured to determine whether or not the own communication terminal is the specified terminal in accordance with whether or not the own communication terminal functions as a relay terminal, wherein the relay function of the own communication terminal is enabled in response to determining through negotiation between the communication terminals that the own communication terminal is a parent node in a communication network to which the communication terminals are connected, wherein the communication network is a network that has a hierarchical network topology, in which only one of the communication terminals functions as a parent communication terminal and the remaining communication terminals function as child communication terminals, in which the transmission and reception of communication messages between child communication terminals are performed via the parent communication terminal, and in which communication terminals that function as the relay terminals are limited to a subset of all communication terminals.