A semiconductor optical modulator comprising: a first semiconductor optical waveguide that has an electro-optic effect, the first semiconductor optical waveguide having a laminated structure including a core layer, a first clad layer, a second clad layer, and a barrier layer, the first clad layer and the second clad layer being disposed below and above the core layer, the first clad layer and the second clad layer which have refractive indexes lower than that of the core layer being made of an n-type semiconductor whose bandgap is wider than that of the core layer, the barrier layer being inserted between the second clad layer and the core layer, the barrier layer in which a potential barrier against electrons is formed being made of a p-type semiconductor; a second semiconductor optical waveguide that is connected to the first semiconductor optical waveguide in a light propagation direction, the second semiconductor optical waveguide having the laminated structure in which the second clad layer has a p-type semiconductor penetrating locally through the n-type semiconductor in a laminated direction in the laminated structure of the first semiconductor optical waveguide; a first electrode that is connected to the first clad layer of the first semiconductor optical waveguide; and a second electrode that electrically connects the second clad layer of the first semiconductor optical waveguide and the p-type semiconductor of the second clad layer of the second semiconductor optical waveguide, characterized by that the second clad layer of the second semiconductor optical waveguide includes the p-type semiconductors at both ends in the light propagation direction, and a plurality of first semiconductor optical waveguides and a plurality of second semiconductor optical waveguides are alternately connected in the light propagation direction. a first semiconductor optical waveguide that has an electro-optic effect, the first semiconductor optical waveguide having a laminated structure including a core layer, a first clad layer, a second clad layer, and a barrier layer, the first clad layer and the second clad layer being disposed below and above the core layer, the first clad layer and the second clad layer which have refractive indexes lower than that of the core layer being made of an n-type semiconductor whose bandgap is wider than that of the core layer, the barrier layer being inserted between the second clad layer and the core layer, the barrier layer in which a potential barrier against electrons is formed being made of a p-type semiconductor; a second semiconductor optical waveguide that is connected to the first semiconductor optical waveguide in a light propagation direction, the second semiconductor optical waveguide having the laminated structure in which the second clad layer has a p-type semiconductor penetrating locally through the n-type semiconductor in a laminated direction in the laminated structure of the first semiconductor optical waveguide; a first electrode that is connected to the first clad layer of the first semiconductor optical waveguide; and a second electrode that electrically connects the second clad layer of the first semiconductor optical waveguide and the p-type semiconductor of the second clad layer of the second semiconductor optical waveguide, characterized by that the second clad layer of the second semiconductor optical waveguide includes the p-type semiconductors at both ends in the light propagation direction, and a plurality of first semiconductor optical waveguides and a plurality of second semiconductor optical waveguides are alternately connected in the light propagation direction.