A method of producing a sensing element used in a detecting device for detecting a target substance in a specimen with the use of plasmon resonance. The method includes the steps of forming an insulating layer on a substrate, the surface of the substrate having an electroconductive layer including a first electroconductive material, forming a plurality of holes through the insulating layer to form an insulating pattern, filling up the holes with a second electroconductive material to form a metallic pattern, removing the insulating pattern, and removing a region other than the region sandwiched by the metallic pattern and the substrate in the electroconductive layer, using the metallic pattern as a mask.