A quantum key distribution protocol is provided that reduces the maximum value of the leaked information amount over the same distance when an adversary makes a photon number splitting attack more than the reduction by the BB84 protocol and the SARG protocol, by making use of the advantages of the BB84 protocol and the SARG protocol. By properly proportioning the existing BB84 protocol and the SARG protocol in accordance with the rate determined by the communication distance between the sender and the receiver of the coherent light, a protocol that is more robust against photon number splitting attack than the known existing protocols can be realized, and long distance quantum key distribution, which was not possible until now, becomes possible.