In a semiconductor device connected to a first optical waveguide, a phase modulation unit, and a second optical waveguide in this order and having an optical modulator guiding light in a first direction, the phase modulation unit includes: a semiconductor layer whose length in the first direction is larger than a width in a second direction orthogonal to the first direction and which is made of monocrystalline silicon; a core part serving as an optical waveguide region formed on the semiconductor layer, and extending in the first direction; a pair of slab parts arranged on both sides of the core part in the second direction; a first electrode coupled with one of the slab parts; and a second electrode coupled with the other of the slab parts. The core part has a p type semiconductor region and an n type semiconductor region extending in the first direction, and the second direction coincides with a crystal orientation of the semiconductor layer.