  The communication distance of QKD is limited by exponential attenuation of
photons propagating through optical fibers. However, it has been shown that
introducing a quantum repeater can improve the order of the attenuation and is
useful for extending the communication distance of QKD when the repeater noise
is ignored. In this paper, we analyze the effectiveness of the quantum repeater
when taking the repeater noise into consideration. We analyze the effectiveness
also from the viewpoint of the security and show that QKD is secure even if a
quantum repeater is used.
