An optical device using an electro-optical element is of great utility value to industry, owing to its fast responsiveness and the like. Prior art has problems such as high cost due to a large number of parts, large device size, and high drive voltage, and it has been a challenge to reduce the number of parts to be used, to downsize the device, and to reduce drive voltage. An optical device of the invention has a reflection structure comprising an input fiber 1 and an output fiber 2 which are fastened by a duplex ferrule 3, a collimating lens 6, and a reflector element 9 for reflecting light. The optical device further comprises a cylindrical lens functioning as a condenser lens 7, and an electro-optical element 8, which is placed near a collection point of the condenser lens 7. Electrodes of the electro-optical element 8 are configured such that the interelectrode distance becomes smaller the closer they are to the collection part.