Monofluorophosphate ester salt, method for producing same, and fluorine ion-releasing composition

[Problem] To provide: a monofluorophosphate ester salt capable of the sustained release of fluorine ions at a significant amount of release; a method for producing the monofluorophosphate ester salt; and a fluorine ion-releasing composition. [Solution] The monofluorophosphate ester salt of the present invention is characterized by being represented by chemical formula (1). (M n+ represents any one species selected from the group consisting of a hydrogen ion, alkali metal ion, alkaline-earth metal ion, transition metal ion, rare-earth element ion, zinc ion, aluminum ion, gallium ion, indium ion, germanium ion, tin ion, lead ion, and onium ion. R 1 represents a hydrocarbon group that has 1 to 20 carbons or a hydrocarbon group that has 1 to 20 carbons and that has at least one of any of a halogen atom, a heteroatom, and an unsaturated bond. n represents the valence.)