  Decoy state method quantum key distribution (QKD) is one of the promising
practical solutions to BB84 QKD with coherent light pulses. In the real world,
however, statistical fluctuations with the finite code length cannot be
negligible, and the securities of theoretical and experimental researches of
the decoy method state QKD so far are based on the asymptotic GLLP's formula
which guarantees only that the limit of eavesdropper's information becomes zero
as the code length approaches infinity. In this paper, we propose a
substantially improved decoy state QKD in the framework of the finite code
length and derive the upper bound of eavesdropper's information in the finite
code length decoy state QKD with arbitrary number of decoy states of different
intensities incorporating the finite statistics. We also show the performance
of our decoy QKD and optimal values of parameters by numerical simulation.
