A plasmonic device and an apparatus and method for generating a surface plasmon polariton (SPP) mode using the plasmonic device are disclosed herein. The plasmonic device includes a first plasmonic waveguide and a second plasmonic waveguide. The first plasmonic waveguide is made of a strip-shaped metal material forming at least one pair of first metal-dielectric interfaces along with a dielectric layer, and extends from an input location to a gap start location. The second plasmonic waveguide is made of a strip-shaped metal material forming at least one pair of second metal-dielectric interfaces in planes identical to those of the at least one pair of first metal-dielectric interfaces of the first plasmonic waveguide, and extends from a gap end location, spaced apart from the gap start location by the length of a gap along the propagation direction of the SPP, to an SPP output location.