Provided are an optical element which can be inexpensively and easily manufactured and which is capable of adequately reducing or removing a higher mode while keeping loss of a fundamental mode low, amethod for manufacturing the same, and an optical modulator. The present invention has a substrate and first through third optical waveguides formed on the substrate and each having a lower claddinglayer, a core layer, and an upper cladding layer, the core layer having a larger refractive index than the lower cladding layer and the upper cladding layer, the first optical waveguide being optically connected to the second optical waveguide, the second optical waveguide being optically connected to the third optical waveguide, the first through third optical waveguides having a mesa structure in which top parts of at least the upper cladding layer and the core layer protrude on the lower cladding layer, the core height of the third optical waveguide being less than the core height of the first optical waveguide, the core height being the height to the top surface of the core layer from bottom surfaces on both sides of the mesa structure or locations where the inclination of lateral surfaces of the mesa structure becomes discontinuous approaching the bottom surfaces, and the mesa width of the third optical waveguide being less than the mesa width of the first optical waveguide, the mesa width being the width of the middle of the core layer.