- This paper proposes a "mechanism-based PKI", in which only a mechanism for generating user's private keys is installed on a smart card. The private key is generated inside the smart card at the event that the legitimate user gives a "seed of private key" to his/her smart card in order to sign a massage. The key exists nowhere except while users are signing a massage. Thus, users no longer need to pay considerable attention to their smart cards. In addition, this paper also proposes a "statistical A/D conversion", which is an effective scheme to convert fingerprint to just one and the same ID in realtime. The statistical A/D conversion enables us to use fingerprint as a seed of private key. We construct an example system for real-time key generation form fingerprint. From some basic experiments that we carried out, the availability of the system is confirmed.