A laser device including a gain medium that has a gain, a waveguide for propagating an electromagnetic wave, and a resonant structure comprised of the waveguide. The gain medium 103 extends in the propagation direction, and is sandwiched, at the top and bottom surfaces in the thickness direction thereof, between a first cladding and a second cladding of negative dielectric constant media. The gain medium is provided with a lateral structure adjacent to at least one of the side surfaces thereof in the width direction perpendicular to the propagation direction and the thickness direction. The lateral structure includes a positive dielectric constant medium which is sandwiched, at its top and bottom surfaces in the thickness direction thereof, between the negative dielectric constant media. The waveguide is comprised of the gain medium 103, the lateral structure, the first cladding and the second cladding.