1. A communication method in an ad-hoc network in which a plurality of communication devices perform ad-hoc communication, comprising: a step in which the communication devices determine a unique communication device in the ad-hoc network; a step in which the communication devices measure hop counts from the unique communication device; and a step in which the communication devices propagate a maximum hop count out of the hop counts, in the ad-hoc network, wherein the communication device periodically transmits specific information, in a range communicatable by the communication device, wherein the specific information includes an identifier specifying a communication device, a hop count from the unique communication device, and a lifetime formed by unifying a lifetime of the information on the identifier and a lifetime of the information on the hop count, wherein the step for determining the unique communication device includes: a step in which a communication device compares a self identifier as an initial value of an identifier to an identifier received from an other communication device, and selects an identifier larger, smaller, or closer to a specific value; and a step in which the communication device retains information on the identifier selected, and transmits the information to an other communication device, wherein the step for measuring the hop count includes: a step in which the unique communication device transmits a hop count included in the specific information starting with ‘0’; a step in which a communication device, when receiving the specific information, adds a given number of hops to the hop count; a step in which a communication device compares the hop count with a given number of hops added, to information on a hop count from the unique communication device, retained until then, and selects a smaller hop count; and a step in which the communication device updates information on the hop count included in the specific information, to the hop count selected, and transmits the information to an other communication device, and wherein the communication device deletes the information on the identifier and the information on the hop count when the lifetime timeouts, wherein the specific information include a maximum hop count, and wherein the step for propagating a maximum hop count out of the hop counts retained by the communication devices, in an ad-hoc network includes: a step for selecting a maximum hop count out of a hop count from the unique communication device, obtained by a communication device in the step for measuring; the maximum hop count received; and information on the maximum hop count retained until then; and a step for updating information on a maximum hop count to a hop count selected by a communication device, for retaining the information, and for transmitting the information to an other communication device, wherein the specific information further includes a lifetime indicating a time of validity of the specific information, wherein an initial value of the lifetime is given by a communication device that is transmitting a maximum hop count, and wherein information on the maximum hop count is deleted when the lifetime timeouts, wherein the lifetime is given by 
 (time required for 1-hop relaying)*(k+n), assuming that k is a value of the maximum hop count and n is a constant.