A wavelength filter includes a first filter circuit and a second filter circuit. The first filter circuit that has a passband that is obtained from a vernier effect by connecting, in series, a plurality of ring resonators each having a different transmission wavelength interval and that is within a gain band of an optical amplifier, and that passes, from the gain band, light at a selected wavelength and light that has a wavelength in a recursive mode and that is produced on a short wavelength side or a long wavelength side of the selected wavelength. The second filter circuit is connected to the first filter circuit in series and suppresses the light at the wavelength in the recursive mode from the light passing through the first filter circuit.