An optical device includes a silicon substrate, and multiple columnar members. The columnar members are integrally formed with the silicon substrate and stand on a top surface of the silicon substrate. The columnar members are made of silicon oxide. Light enters the columnar members in a first direction and propagates through the columnar members in a second direction. The columnar members extend in a third direction. The columnar members are arranged with a gap in the second direction. The second direction is perpendicular to the third direction. An angle between the first and third directions is greater than a critical angle and equal to or less than 90 degrees.