The object is to provide a highly reliable, high-performance, and low-price tunable light source and the like. The present invention comprises: a multiple resonator that oscillates with a wavelength where frequencies of three or more resonant elements with shifted cycles intersect; and a tunable device for controlling the resonant wavelength of the multiple resonator by simultaneously changing the respective optical path lengths of the plurality of resonant elements constituting the multiple resonator. The multiple resonator has a structure in which three resonant elements are connected in series; and provided that the optical path lengths of the resonant elements are L0, L1, L2, Vernier orders are M1>1, M2>1, the optical path lengths are defined as L 1 = M 1 M 1 - 1 ⁢ L 0 , ⁢ L 2 = M 2 M 2 - 1 , phase amount “Phase” is the changed optical path length standardized by one-wavelength of the optical path length, and the respective phase amount “Phase” of the two ring resonant elements whose optical path lengths are to be changed are defined as “PhaseM1” and “PhaseM2”, the tunable device controls the increase/decrease amount of the phase amount “PhaseM1” and “PhaseM2” based on a linear function with a slope of M 2 - 1 M 1 - 1 .