A wavelength tunable filter and a wavelength tunable laser module are a codirectional coupler type whose characteristics do not vary significantly with a process error. They are structured so as to include a semiconductor substrate which has a first optical waveguide and a second optical waveguide. The first and the second optical waveguides are extended from a first side of the semiconductor substrate to an opposing second side thereof. The first optical waveguide includes a first core layer, which has a planar layout having periodic convexes and concaves, and a pair of electrodes, which vertically sandwich the first core layer. The second optical waveguide includes a second core layer, which has a lower refractive index than the first core layer. Further, a layer having the same composition and film thickness as the second core layer is placed under the first core layer.