The present invention aims to provide a tuning light source apparatus including a multiple-optical resonator, where the resonance frequencies of the respective optical resonators in a multiple-optical resonator are exactly coincided with the set frequency, and the frequency of the output laser beam is locked within a range of about 1 GHz from the set frequency. Current is flowed to TO phase shifters based on lights detected by light receiving elements, and the resonance wavelengths of resonators are adjusted in an aim of obtaining a state in which an intensity of an oscillation light becomes a maximum and at the same time an intensity of a light from a through port becomes a minimum.