Secure key in BB84 quantum key distribution system is affected by the state preparation flaws, which arise mainly from the errors in the applied voltage to the phase modulators. We propose dual-parallel modulators(DPM) as robust phase modulators for the state preparation. The phase error in a DPM increases as a quadratic function of the voltage error, whereas the error increases linearly in a conventional phase modulator and a dual-drive modulator. We measured the visibility of the interference at the receiver and experimentally confirmed the superiority of the DPMs on the robustness to the applied voltage error.