Abstract: A risk of occurrence of folds, cracks, peeling, and the like in a heater electrode pattern is avoided beforehand by forming a thin-film heater electrode pattern on a polymer optical waveguide with good adhesiveness, and minimizing surface damage of the polymer optical waveguide due to an electrode forming process. According to an electrode forming method for a polymer optical waveguide, after the surface of the polymer optical waveguide is subjected to activation processing, a heater electrode film is formed on the entire surface thereof. A resist is then applied onto the heater electrode film, to form a resist pattern by photolithographic processing in which a heater electrode is patterned. The resist pattern is etched to form a heater electrode pattern, and then remaining resist on the heater electrode pattern is removed.