A wavelength locker includes a splitter configured to split a portion of light that is output from a light source into a first monitoring light and a second monitoring light; a first periodic filter configured to receive the first monitoring light; a second periodic filter configured to receive the second monitoring light; a first optical receiver configured to monitor an intensity of light that has passed through the first periodic filter; and a second optical receiver configured to monitor an intensity of light that has passed through the second periodic filter, wherein the first periodic filter and the second periodic filter each has a wavelength characteristic in which an intensity of transmitted light changes periodically with respect to a wavelength, and the wavelength characteristic of the first periodic filter and the wavelength characteristic of the second periodic filter shift in opposite wavelength directions when temperature changes.