To provide a coating agent capable of forming a coating film having high hardness, high rubbing resistance, and strong adhesibility, without necessitating a high-temperature treatment as in a conventional method, even when titanium oxide having a high refractive index is used and a process for producing the coating agent; a coating film formed by applying the above-mentioned coating agent and a process for producing the coating film; and an optical instrument comprising the above-mentioned coating film and a process for producing the optical instrument.A coating agent for an optical instrument, obtainable under the conditions where one or more titanium sources selected from the group consisting of titanium alkoxides and titanium salts, a nitrogen-containing organic base, and water are co-present; a coating agent for an optical instrument, obtainable by mixing a hydrolyzed solution of one or more titanium sources selected from the group consisting of titanium alkoxides and titanium salts, and a nitrogen-containing organic base; and a coating agent for an optical instrument, obtainable by hydrolyzing one or more titanium sources selected from the group consisting of titanium alkoxides and titanium salts, in the presence of a nitrogen-containing organic base.