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.