The present invention relates to an apparatus and method for performing optical Differential Phase Shift Keying (DPSK) modulation, which transmits a phase difference between adjacent bits as information. The electrically band-limited optical DPSK modulating apparatus includes a light source, a Non-Return-to-Zero (NRZ) signal generator, a pre-coder, an electrical low pass filter, and a phase modulator. The NRZ signal generator generates a NRZ signal. The pre-coder codes the NRZ signal generated by the NRZ signal generator into a differential signal. The low pass “filter electrically limits a bandwidth of the differential signal obtained through the pre-coder. The phase modulator modulates an optical signal input from the light source into a DPSK signal using the differential signal having the bandwidth electrically limited by the low pass filter.