An optical transmission system includes a plurality of optical nodes that transmits wavelength multiplexing light including a plurality of signal light components having different wavelengths, wherein each of the optical nodes includes superimposed signal light generation circuit which superimposes a low frequency signal having a common frequency on a corresponding signal light component included in the wavelength multiplexing light; low frequency signal extraction circuit which extracts a low frequency signal having a frequency of a given range from a corresponding signal light component; and pass-through node number measurement circuit which measures, for each of the signal light components, a pass-through node number based on the frequency of the low frequency signal extracted by the low frequency signal extraction circuit, the pass-through node number being the number of optical nodes through which the signal light component has passed before being transmitted to a specific optical node.