An optical device includes: a silicon substrate; a plurality of silicon oxide columns having a rectangular plan shape; and a cavity disposed between the columns. Each column has a lower portion disposed on the substrate. Each column has a width defined as W1. The cavity has a width defined as W2. A ratio of W1/W2 becomes smaller as it goes to the lower portion of the column. A core layer provided by the columns and the cavity can have the thickness equal to or larger than a few dozen .mu.m easily. Therefore, connection loss between a light source and the device is reduced.