An optical modulator, a substrate for an optical modulator, a method of manufacturing an optical modulator, and a method of manufacturing a substrate for an optical modulator that can reduce a propagation loss are provided. An optical modulator 1 according to an embodiment includes: a base substrate 10; a waveguide substrate 20 disposed over the base substrate 10 and including an electro-optic effect; a waveguide 23 formed on the waveguide substrate 20 for performing optical modulation; and an electrode 40 configured to apply a voltage to the waveguide 23. Here, the base substrate 10 and the waveguide substrate 20 are made of the same material, the waveguide 23 is formed inside the waveguide substrate 20, and a refractive index of the waveguide substrate 20 is larger than a refractive index of the base substrate 10.