When light is made incident to an optical waveguide path 14 formed through a main body 12 composed of silver (Ag) that is a plasmon active medium, surface plasmon is generated on a definition face 55 of the optical waveguide path 14 (including a fine aperture 16). Thus, the intensity of the light propagating in the optical waveguide path 14 is strengthened as the light propagates toward the fine aperture 16. In addition, a distal end 51 of a first protrusive piece 13a is more protrusive as compared with a distal end 52 of a second protrusive piece 13b. Thus, in a distal end part of a projection 13, the light is focused in the vicinity of the first protrusive piece 13a based on an intensity distribution of an electric field at the distal end part. Thus, the light having seeped out from the fine aperture 16 is restricted from spreading in a polarizing direction. Therefore, the light intensity of the light propagated through the optical waveguide path 14 is increased at a low cost without causing an increase of the spread of the light while maintaining a good S/N ratio.