A plasmon sensor includes a supporter, a first metal layer on a lower surface of the first supporter, and a second metal layer having an upper surface facing a lower surface of the first metal layer. A hollow space is provided between the first and second metal layers, and is configured to be filled with a test sample containing a medium. The area of the first metal layer is smaller than the area of the upper surface of the supporter. The supporter has a region having hydrophilic property and facing and contacting the hollow space. This the plasmon sensor has a small size and a simple structure.