A sensor including a metal oxide film formed by wet processing and capable of detecting light, hydrogen gas, and air pressure is provided. A sensor according to the present invention is a sensor capable of detecting light, hydrogen gas, and air pressure, the sensor including a metal oxide film produced by a process including the steps of: (a) forming an organic film by using a primer composition containing (i) an addition polymerizable compound including three or more reactive groups, (ii) an addition polymerizable compound including an acid group, and (iii) an addition polymerizable compound including a hydrophilic functional group; (b) forming a metal (M1) salt from the acid group; (c) substituting the metal (M1) salt of the acid group with a metal (M2) salt by treating the organic film with a metal (M2) ion aqueous solution; (d) reducing the metal (M2) ion so that a metal film is formed on a surface of the organic film; and (e) oxidizing the metal film.