In each of a plurality of submarine observation apparatus (1 to n), a branching unit (63) branches fixed-wavelength light (.lamda.1) from an incoming wavelength-multiplexed light signal. An observation signal modulating unit (64) modulates the intensity of the branched fixed-wavelength light (.lamda.1) with observation information multiplexed by an observation signal multiplex unit (61). A combining unit (65) combines light signals (.lamda.2) to (.lamda.n) passing through the branching unit (63) and the fixed-wavelength light (.lamda.1a) modulated by the observation signal modulating unit (64) into a composite light signal, and outputs it to an optical fiber (12a). Therefore, in each of the plurality of submarine observation apparatus (1 to n), there is no necessity for providing a wavelength-division-multiplexing-transmission optical transmitter which requires high wavelength stability.