  In this paper, we discuss the security of the differential-phase-shift
quantum key distribution (DPSQKD) protocol by introducing an improved version
of the so-called sequential attack, which was originally discussed by Waks et
al. Our attack differs from the original form of the sequential attack in that
the attacker Eve modulates not only the phases but also the amplitude in the
superposition of the single-photon states which she sends to the receiver.
Concentrating especially on the "discretized gaussian" intensity modulation, we
show that our attack is more effective than the individual attack, which had
been the best attack up to present. As a result of this, the recent experiment
with communication distance of 100km reported by Diamanti et al. turns out to
be insecure. Moreover it can be shown that in a practical experimental setup
which is commonly used today, the communication distance achievable by the
DPSQKD protocol is less than 95km.
