To provide an inexpensive multiwavelength light source that can easily change a wavenumber interval over a wide range.

The multiwavelength light source is provided with a first optical coupler 2b that separates revolving light and incident light that are optically combined like a ring by a fiber, an optical isolator 3 that regulates the advancing direction of the revolving light in one direction, an optical amplifier 4 for amplifying the revolving light, a non-linear optical modulator 5 for modulating the revolving light in multiwavelength, a wavelength filter 6 for selectively transmitting the wavelength element of the revolving light, and a second optical coupler 2a for synthesizing the revolving light and species laser light, and it comprises a laser 1 for making the species laser light enter the second optical coupler 2a, a high frequency source 7 for driving the non-linear optical modulator 5, and an amplifier 8 that is arranged between the high frequency source 7 and the non-linear optical modulator 5 to amplify a high-frequency signal.