A wavelength tunable light source includes a light source, a wavelength monitor circuit configured to receive light emitted from the light source, and a processor that controls the light source based upon an output value of the wavelength monitor circuit, wherein the wavelength monitor circuit has a wavelength filter with a periodic transmission spectrum, and three photo detectors connected to outputs of the wavelength filter, and wherein the processor is configured to calculate a ratio of photo-detection normalized with two of three quantities of light received at the three photo detectors and control electric current input to the light source such that a calculated ratio of photo-detection approaches a target ratio at a designated wavelength.