We have proposed a decentralized Peer-to-Peer (P2P) cloud storage scheme achieving high confidentiality by using a large number of unspecified P2P nodes. There is one crucial problem in the scheme: it is generally difficult to maintain the reliability of the stored data since there is no designated central server which supervises the whole system, and hence, it will be infeasible to certainly restore the stored data if the P2P nodes are to be less reliable. In this paper, we propose a method to maintain the high reliability of the stored data by applying mutual monitoring and autonomous recovering operations among the P2P nodes. Using the method in combination with the conventional scheme, one can achieve a high reliability P2P cloud storage with high confidentiality. We then give a quantitative evaluation of the reliability on the stored data in the proposed storage.