A wavelength-division multiplex optical transmission system is provided with: preliminary optical modulators 111 to 11n for outputting optical signals having different wavelengths from each other after being modulated by communications signals 11 to 1n that are signals to be supplied to specific optical receiving parts; an optical fiber 510 for transmitting the multiplexed optical signal; a subsequent optical modulator 210 for collectively modulating the transmitted optical signal so as to collectively modulate the optical signals being multiplexed by a broadcast signal 20 that is to be supplied equally to all optical receiving parts; and an optical fiber 520 for transmitting the modulated optical signal. A frequency band for the broadcast signal 20 is set not to overlap with any of those of the communications signals 11 to 1n. Accordingly, in each of optical receiving parts 411 to 41n, an electrical signal in which the communication signal 11 to 1n and the broadcast signal 20 are frequency-multiplexed can be obtained.