Quantum key distribution (QKD) allows two parties to share an unconditional secret key. The first QKD protocol has been proposed by Bennett and Brassard in 1984, which is called BB84. The practical BB84 is vulnerable against photon-number splitting (PNS) attacks, however, it is shown that this problem can be solved by utilizing information from a built-in decoy state. In order to prepare a decoy state, it seems to need an additional complexity to the experimental setup, such as the random amplitude modulation for weak coherent pulse scheme. Here, we propose an efficient QKD protocol based on photon-pair generation from parametric down-conversion, which uses the only different post-processing of the classical data from the conventional protocol. Assuming the use of practical detectors, we analyze the unconditional security of the new scheme, and show that it improves the secure key generation rate by several orders of magnitude at long distances.