  Differential-phase-shift (DPS) quantum key distribution stands as a promising
protocol due to its simple implementation, which can be realized with a train
of coherent pulses and a passive measurement unit. Besides, this protocol has
the advantage of being robust against imperfections in the light source.
Unfortunately, however, as for the measurement unit, existing security proofs
put unrealistic assumptions on it, which could be security loopholes in actual
implementations. In this paper, we enhance the implementation security of the
DPS protocol by incorporating a major imperfection in the measurement unit.
Specifically, our proof enables us to employ practical beam splitters with a
known range of the transmittance rather than the one with exactly $50\%$, as
was assumed in the existing security proofs. Our numerical simulations
demonstrate that even with fluctuations of $\pm0.5\%$ in the transmittance from
the ideal value, the key rate degrades only by a factor of 0.57. This result
highlights the feasibility of the DPS protocol with practical measurement
setups.
