Abstract: A film-coated sensor is formed by providing an electrically conductive substrate (2), which has a sectional area of less than 10.sup.-5 cm.sup.2, connected to a lead wire (5) at one end of an insulative capillary (1) and inscribed in the capillary (1), and providing a coating film (7) at another end, which is open, of the capillary (1) by an electrolytic polymerization process.The film-coated sensor obtained has a surface which is uniform, the film coating is easy to control, and since the sensor is not readily susceptible to flow of a specimen fluid, there is little drift and response is quick. Owing to its small size, the sensor is capable of measuring very small samples and of performing measurement upon being directly inserted into a living body or tissue.The sensor can be used also as an enzyme sensor by coating the surface of the film-coated sensor with a fixing enzyme film.