To improve receiving sensitivity by compressing a signal light pulse by performing phase modulation and wavelength dispersion compensation at the time of high-density wavelength division multiplexing. There are provided an optical phase modulator and a wavelength dispersion module, for performing signal processing to an optical signal propagating from the transmission side through an optical fiber transmission path and received on the receiving side to thereby obtain communication information. By the optical phase modulator, a frequency chirp corresponding to a phase modulating signal is applied to the optical signal received on the receiving side. By the wavelength dispersion module, wavelength dispersion is applied to the optical signal which is frequency-chirped by the phase modulator, so that the optical signal is pulse-compressed.