A surface plasmon polariton modulator capable of locally varying a physical property of a dielectric material to control a surface plasmon polariton. The surface plasmon polariton modulator includes a dielectric layer, including first and second dielectric portions, which is interposed between two metal layers. The second dielectric portion has a refractive index which varies with an electric field, a magnetic field, heat, a sound wave, or a chemical and/or biological operation applied thereto. The surface plasmon polariton modulator is configured to control one of an advancing direction, an intensity, a phase, or the like of a surface plasmon using an electric signal. The surface plasmon polariton modulator can operate as a surface plasmon polariton multiplexer or a surface plasmon polariton demultiplexer.