In this talk, I will discuss the unconditional security of Quantum Key Distribution (QKD) Protocol, especially SARG04 protocol prososed by Scarani et al. This protocol uses the same four states as in BB84 and differs only in the classical postprocessing protocol. It is expected in SARG04 protocol that we can distill a secure key not only from sigle-photon part but also two-photon part, which is a big difference from BB84 protocol. I will explain how to prove the unconditional security of SARG04 and show that we can indeed distill a secure key from two-photon part. Finally, we compare the security of SARG04 to the one of BB84 assuming a depolarizing channel and practical experimental setup as well.