A new and novel sensor having a simple structure with high detection sensitivity. The sensor (S1) includes the following from measuring light (L1) input side in the order listed below: a first reflector (10) having semi-transmissive and semi-reflective properties; a translucent body (20); and a second reflector (30) having perfect reflection properties, or semi-transmissive and semi-reflective properties. The first reflector (10) and/or second reflector is brought into contact with a specimen, and the average complex refractive index varies with the specimen. Absorption properties for absorbing light having a particular wavelength are produced by these components, the properties of the measuring light (L1) are changed by the optical properties including the absorption properties, the output light (L2) is outputted from the first reflector (10) and/or second reflector (30), and the physical properties of the output light (L2) that vary according to the optical properties are detected.