An optical modulator and an optical transmission device using the same are provided. The optical modulator includes an optical waveguide substrate where an optical waveguide is formed; a light modulation element, provided in the optical waveguide substrate and including a modulation electrode applying an electric field corresponding to a modulation signal to the optical waveguide; a terminal substrate, disposed near the light modulation element and including terminal resistors that terminate the modulation signal, wherein the optical waveguide substrate, the light modulation element and the terminal substrate are accommodated in a housing; and a heat dissipation auxiliary element, provided between the terminal resistors and the housing. A distance b between the terminal resistors and the heat dissipation element is set to be shorter than a distance a from the terminal resistors to an end on the side of the optical waveguide substrate of the terminal substrate.