A photoconductive device that generates or detects terahertz radiation includes a semiconductor layer; a structure portion; and an electrode. The semiconductor layer has a thickness no less than a first propagation distance and no greater than a second propagation distance, the first propagation distance being a distance that the surface plasmon wave propagates through the semiconductor layer in a perpendicular direction of an interface between the semiconductor layer and the structure portion until an electric field intensity of the surface plasmon wave becomes 1/e times the electric field intensity of the surface plasmon wave at the interface, the second propagation distance being a distance that a terahertz wave having an optical phonon absorption frequency of the semiconductor layer propagates through the semiconductor layer in the perpendicular direction until an electric field intensity of the terahertz wave becomes 1/e2 times the electric field intensity of the terahertz wave at the interface.