A fluorescent probe for measuring magnesium ion, which can selectively form a complex with magnesium ion in aqueous system is disclosed. The fluorescent probe for measuring magnesium ion according to the present invention has the structure represented by the following Formula [I]: ##STR00001## (wherein R.sup.1 represents a hydrogen atom, metal atom or an ester-forming group; A represents a group which forms a ring structure together with carbon atom 1 and carbon atom 2; and X is a fluorescent group which may form a condensed ring together with the ring containing the group A).