To provide a heterodyne light spectrum analyzer capable of correcting a non-uniform sweep speed of a local oscillation source.

This light spectrum analysis system comprises: a local oscillation source 505 for generating a sweep local oscillator signal (local oscillation signal); an optical coupler 510 for outputting an optical signal by combining an input signal with the local oscillation signal; a heterodyne receiver 512 for sending out a heterodyne beat signal by inputting the combined optical signal; a relative frequency measurement system 520 for generating measurement local oscillator frequency sweep speed information (speed information) by responding to the local oscillation signal; and a signal processor 516 for generating an output signal expressing an optical parameter of the input signal from the heterodyne beat signal and the speed information.