A sensor device has an optical waveguide containing an electro-optic crystal for propagating light, a coupler provided adjacent to the optical waveguide to propagate a terahertz wave generated from the electro-optic crystal as a result of the propagation of light in the optical waveguide, and a detector for detecting the terahertz wave propagating through the coupler or the light propagating through the optical waveguide. The terahertz wave is totally reflected in a section of the coupler opposite to a section where the coupler is adjacent to the optical waveguide while passing through and propagating in the optical waveguide, and in the total reflection section, the terahertz wave interacts with a subject placed close to the total reflection section.