A detection device for detecting an analyte which may be contained in a specimen, comprising: a plurality of metallic nanoparticles (11, 12) each modified with a host molecule (13, 14) allowing said analyte to specifically adhere thereto; a substrate comprising an upper surface, the substrate configured to hold a liquid containing said specimen and said plurality of metallic nanoparticles (11, 12), such that a gas-liquid interface of the liquid and a gas surrounding the liquid is formed; a first light source configured to emit polarized light for assembling said plurality of metallic nanoparticles (11, 12) together, thereby forming a metallic nanoparticle assembly; an objective lens configured to focus and introduce said polarized light into said liquid; a photoreceiver configured to receive light from said liquid; and a detector configured to detect said analyte based on a signal received from said photoreceiver; characterized in that the detection device comprises an adjustment mechanism comprising a control unit, wherein the adjustment mechanism is configured to positionally adjust either a stage the substrate is mounted upon or the objective lens, in order to adjust a distance between said objective lens and a gas-liquid interface of said liquid and a gas surrounding said liquid so that said objective lens has a focal point in said liquid in a vicinity of said gas-liquid interface, said vicinity being a position closer to gas-liquid interface than a midpoint between the upper surface of substrate and gas-liquid interface on an optical axis of said polarited light; and wherein the control unit is configured to calculate a distance from the upper surface of substrate to the focal point. a plurality of metallic nanoparticles (11, 12) each modified with a host molecule (13, 14) allowing said analyte to specifically adhere thereto; a substrate comprising an upper surface, the substrate configured to hold a liquid containing said specimen and said plurality of metallic nanoparticles (11, 12), such that a gas-liquid interface of the liquid and a gas surrounding the liquid is formed; a first light source configured to emit polarized light for assembling said plurality of metallic nanoparticles (11, 12) together, thereby forming a metallic nanoparticle assembly; an objective lens configured to focus and introduce said polarized light into said liquid; a photoreceiver configured to receive light from said liquid; and a detector configured to detect said analyte based on a signal received from said photoreceiver; characterized in that the detection device comprises an adjustment mechanism comprising a control unit, wherein the adjustment mechanism is configured to positionally adjust either a stage the substrate is mounted upon or the objective lens, in order to adjust a distance between said objective lens and a gas-liquid interface of said liquid and a gas surrounding said liquid so that said objective lens has a focal point in said liquid in a vicinity of said gas-liquid interface, said vicinity being a position closer to gas-liquid interface than a midpoint between the upper surface of substrate and gas-liquid interface on an optical axis of said polarited light; and wherein the control unit is configured to calculate a distance from the upper surface of substrate to the focal point.