Sensing Device

[Problem] To provide a small-sized, stable, and highly sensitive detection device by achieving an optical system which is the most suitable for an optical waveguide mode sensor using a spectral measurement method. [Solution] A detection device including: a detection plate in which a silicon layer and a silicon oxide layer are arranged in this order on a silica glass substrate; an optical prism which is optically adhered to a surface of the silica glass substrate of the detection plate, where the surface is not provided with the silicon layer and the silicon oxide layer; a light-irradiation unit configured to irradiate light to the detection plate through the optical prism and arranged so that light is incident on the optical prism with a fixed incident angle; and a light-detection unit configured to detect intensity of reflected light reflected from the detection plate, wherein the detection device detects a change in dielectric constant in the proximity of the surface of the silicon oxide layer of the detection plate by detecting a change in property of the reflected light, and wherein in the optical prism, an angle between an incident surface on which light irradiated from the light-irradiation unit is incident and an adhesion surface which adheres to the detection plate is 43° or less.