A wavelength tunable laser according to the present invention includes a first facet and a second facet opposite the first facet, a reflective region provided adjacent to the second facet, and a gain region provided between the first facet and the reflective region. The reflective region has a plurality of reflection peak wavelengths that periodically vary at a predetermined wavelength interval. The first facet and the reflective region constitute a laser cavity. Furthermore, the gain region includes an active layer where light is generated, a diffraction grating layer having a diffraction grating whose grating pitch varies in a light propagation direction, a refractive-index control layer provided between the active layer and the diffraction grating layer, a first electrode for injecting current into the active layer, and a plurality of second electrodes arranged in the light propagation direction to inject current into the refractive-index control layer.