To provide a light-based sensor that can accurately and widely measure a position to be measured being located extremely near the sensor by using a solid light guide.

The sensor 1 for sensing the distance to a medium and/or the displacement and physical characteristics of the medium has a housing 2, at least a pair of solid light guides 8 and 19 that have input and output end surfaces, at least one light source 12, and at least one light intensity measurement means 18. Then, the light source 12 and the light intensity measurement means 18 are optically interlocked to the input end surface of the first light guide 8 and the output end surface of the second light guide 10, respectively, and the light guides 8 and 10 are optically insulated each other. However, surfaces 14 and 16 of the light guides 8 and 10 are arranged at least adjacently each other, the surfaces 14 and 16 are cut to the optical axis of waveguides 8 and 10 at an angle of θ, and an angle of α is formed between the waveguides 8 and 10, thus enabling light being emitted from the output end surface of the light guide 8 to enter a thin film 4 at an incident angle of β=f (α, θ) on operation.