To provide a differential phase shift keying light transmitting system that suppresses a bit error even if a pulse spreading occurs.

In the light transmitting system of a differential phase shift keying light transmitting method, the system is configured to make a polarization state of adjacent two pulses perpendicular in a transmitter (300), and to multiplex same slots in the polarization state with each other in a receiver (310). Thus, even if an optical pulse width widens to overlap adjacent optical pulses because of a wavelength dispersion or the other factors during propagation of an optical fiber (307), the bit error caused by the unwanted overlapping does not occur.