A method for manufacturing a semiconductor optical modulator includes forming a p-type semiconductor layer on a main surface of a p-type semiconductor substrate; forming a pair of stripe-shaped masks on the p-type semiconductor layer, the stripe-shaped masks extending in a first direction along the main surface of the p-type semiconductor substrate and being spaced apart from each other; simultaneously forming a hole and a pair of stripe structures extending in the first direction by etching the p-type semiconductor layer through the stripe-shaped masks, the pair of stripe structures defining the hole; after removing the stripe-shaped masks, forming a buried layer in the hole; forming a core layer on the buried layer and the stripe structures; and forming an upper cladding layer on the core layer. The buried layer is made of a semiconductor material with a lower optical absorption loss than that of the p-type semiconductor layer.