To provide a diffractive optical element that allows simplification of a manufacturing process and reduction in manufacturing costs. A diffractive optical element includes a first transparent substrate having a pair of transparent interdigitated electrodes facing each other formed on its surface, a second transparent substrate, and a liquid crystal sandwiched therebetween. Alignment layers are formed on surfaces of the first and second transparent substrates facing the liquid crystal. Pads are provided on the transparent electrodes and electrically connected to an outside. A seal material and a liquid crystal sealing port are provided on the first transparent substrate. The seal material is formed into a partly cut rectangle, and the liquid crystal sealing port is formed at the cut portion of the rectangular seal material.