In this paper, we present a backup technique for Peer to Peer applications, such as a distributed asynchronous Web-Based Training system that we have previously proposed. In order to improve the scalability and robustness of this system, all contents and functions are realized on mobile agents. These agents are distributed to computers, and using a Peer to Peer network that modified Content-Addressable Network they can obtain. In the proposed system, although entire services do not become impossible even if some computers break down, the problem that contents disappear occurs with an agent's disappearance. As a solution for this problem, backups of agents are distributed to computers. If failures of computers are detected, other computers will continue service using backups of the agents belonging to the computer. The developed algorithms are examined by experiments.