Provided herein is a semiconductor optical device, including a waveguide including lattices buried therein and having a buried hetero (BH) structure formed in an optical oscillation region in which single mode light is oscillated, a waveguide having a deep ridge structure formed in an optical modulation region, and a passive waveguide formed in a mode transition region interposed between the optical oscillation region and the optical modulation region, formed as a connecting structure of the waveguide having the BH structure extending from the optical oscillation region and the waveguide having the deep ridge structure extending from the optical modulation region, and inducing evanescent optical coupling, wherein a width of the waveguide having the BH structure in the mode transition region is smaller than a width of the waveguide having the deep ridge structure in the optical modulation region.