Provided are an optical sensor and an analyzer, including an optical sensor section in which a cladding layer of an optical fiber is removed so as to expose a core layer by a predetermined optical path length, and a protective material is added to a surface of the exposed core layer, the protective material having higher resistance to an organic solvent, base, or acid than that of the cladding layer; a light source device that causes light to enter one end of the optical fiber; a light receiving device that receives transmitted light emitted from another end of the optical fiber; and a control device that controls the light source device and the light receiving device to measure optical transmittance in the optical sensor based on a ratio of intensity of the light emitted from the light source device to intensity of the light received by the light receiving device.