A sensor for detecting looseness of a plurality of bolts and nuts fixing parts includes: a non-conductive film attached to a surface of the part, which is one of the parts, including holes H from which the bolts are inserted; and a conductive film attached to the non-conductive film, configured to receive supply of a high-frequency signal, and having a same planar shape as that of the non-conductive film, wherein the planar shape is a shape in which a resonance frequency of the high-frequency signal confined in the non-conductive film is different according to a position of a loosened bolt, and the planar shape of the conductive film is a shape in which a distance between a bolt and an end portion of the conductive film is different for each of the plurality of bolts.