An optical device includes an optical waveguide that is constituted of a cladding and a core formed on a substrate. The optical waveguide includes an input waveguide, an attenuator section that is connected to the input waveguide, a removing section that is connected to the attenuator section, and an output waveguide that is connected to the removing section. The attenuator section is configured to have more waveguide modes that are guided on a cross section perpendicular to a traveling direction of light in a connecting portion connected to the removing section compared to a connecting portion connected to the input waveguide. The removing section is configured to have less waveguide modes that are guided on a cross section perpendicular to a traveling direction of light in a connecting portion connected to the output waveguide compared to a connecting portion connected to the attenuator section.