An optical device 1 has a photodetecting element 6, an optical waveguide layer 4 containing core portions 4a to 4c extending in directions crossing a layer thickness direction and a clad portion 4h covering the core portions 4a to 4c, and an optical waveguide substrate 2 having the end face 4g of the core portion 4c optically coupled to the photodetecting element 6 on the side surface 2a thereof. The optical waveguide substrate 2 has substrates 3 and 5 disposed so that the principal surfaces 3a and 5a face each other. The optical waveguide layer 4 is provided between the substrate 3 and the substrate 5. The photodetecting element 6 is mounted on the mount area 2c on the side surface 2a of the optical waveguide substrate 2. The mount area 2c is set to contain the end face 4g of the core portion 4c, a part of the side surface 3b of the substrate 73 and a part of the side surface 5b of the substrate 5. Accordingly, in the optical device, the optical coupling efficiency between a semiconductor optical element such as a light emission element and a photodetecting element and an optical waveguide can be enhanced.