Abstract: A scheme for depositing a private key used in the RSA cryptosystem which is capable of maintaining the private key more safely, without requiring a user to always carry around a storage medium. In this scheme, a private key of a user is divided into a first partial private key and a second partial private key at a user's entity, where the first partial private key is set to be maintained at the user's entity, while the second partial private key is deposited from the user's entity to the other entity. Then, the second partial private key is delivered from the other entity to the user's entity in response to a request from the user's entity, and the first partial private key maintained at the user's entity and the second partial private key delivered from the other entity are composed so as to obtain the private key to be used in a processing according to the RSA cryptosystem at the user's entity. This scheme can be generalized to a case of dividing the private key into a plurality of partial private keys.