A sensing system using a sensing element (10, 210) being constituted by a transparent body (11, 211) sandwiched by first and second reflectors (11, 211, 13, 213) one or each of which is in contact with a specimen (17, 217), and exhibiting an absorption characteristic varying with the specimen. The first reflector is a partially transparent reflective, and the second reflector is completely reflective, or partially transparent reflective. A light injection unit (20, 20-11) injects light onto the first reflector, and a light detection unit (30, 233) detects the intensity of light outputted fromthe sensing element in response to the injection. The light injection unit has a wavelength stabilizing arrangement and injects laser light, or injects light at two wavelengths. In the latter case, the light detection unit detects the intensities of outputted light at the two wavelengths, and a calculation unit (240) obtains the difference between the intensities.