A semiconductor optical modulator includes a first n-type semiconductor region, a first p-type semiconductor region, an i-type semiconductor region, a second p-type semiconductor region, and a second n-type semiconductor region that constitute a stacked layer structure. The stacked layer structure includes a first cladding layer, a second cladding layer, and a core layer disposed between the first and second cladding layer. The first n-type semiconductor region and the first p-type semiconductor region form a first p-n junction disposed in an intermediate region between the first and second cladding layer. The second p-type semiconductor region and the second n-type semiconductor region form a second p-n junction disposed in the intermediate region or the second cladding layer. The intermediate region, the first n-type semiconductor region, and the second n-type semiconductor region include the core layer, the first cladding layer, and part or all of the second cladding layer, respectively.