A mixture of an Fe ion detection agent comprising a compound represented by formula (fluorescence probe): wherein R 1 and R 2 are the same or different and represent lower alkyl, carboxy lower alkyl, aryl, or aryl lower alkyl; R 3 and R 4 are the same or different and represent hydrogen, halogen, or lower alkyl; R 1 and R 2 may be taken together with the nitrogen atom to which they are attached to form a pyrrolidine ring, a piperidine ring, a morpholine ring, or an optionally substituted piperazine ring; R 1 and R 3 may be taken together to form trimethylene; R 2 and R 4 may be taken together to form trimethylene; R 5 represents optionally protected hydroxyl, or a group represented by formula: -NR 51 R 52 wherein R 51 and R 52 are the same or different and represent lower alkyl, carboxy lower alkyl, aryl, or aryl lower alkyl, or R 51 and R 52 may be taken together with the nitrogen atom to which they are attached to form a pyrrolidine ring, a piperidine ring, a morpholine ring, or an optionally substituted piperazine ring; R 6 and R 7 are the same or different and represent hydrogen, halogen, or lower alkyl; when R 5 is the group represented by formula, R 51 and R 6 may be taken together to form trimethylene, or R 52 and R 7 may be taken together to form trimethylene; ring A represents an optionally substituted aromatic ring or an optionally substituted heteroaromatic ring; V represents O or SiR 10 R 11 wherein R 10 and R 11 are the same or different and represent hydrogen or lower alkyl; W represents CH 2, CO, or S p; Z represents O or NR 9 wherein R 9 represents hydrogen or alkyl; m and n are the same or different and represent 0 or 1; and p represents 1 or 2, in combination with a compound having the same or different 3 to 5 coordinating positions selected from the group consisting of amino, carboxyl, and pyridine ring, wherein the compound is selected from pyridylmethyl amine compounds and aminocarboxylic acid compounds and salts of these compounds,or in combination with a compound selected from N,N,N',N'-ethylenediaminetetraacetic acid (EDTA), 1,3-propanediamine tetraacetic acid (PDTA), ethylenediamine triacetic acid (HEDTA), 1,2-diaminocyclohexane tetraacetic acid (CyDTA), N,N-bis(2-hydroxybenzyl) ethylenediamine diacetic acid (HBED), ethylenediamine disuccinic acid (EDDS), hexamethylenediamine tetraacetic acid (HDTA), diaminopropane tetraacetic acid (Methyl-EDTA) and salts of these compounds, wherein the compound represented by formula (fluorescence probe) and the compound having coordinating positions are mixed. R 1 and R 2 are the same or different and represent lower alkyl, carboxy lower alkyl, aryl, or aryl lower alkyl; R 3 and R 4 are the same or different and represent hydrogen, halogen, or lower alkyl; R 1 and R 2 may be taken together with the nitrogen atom to which they are attached to form a pyrrolidine ring, a piperidine ring, a morpholine ring, or an optionally substituted piperazine ring; R 1 and R 3 may be taken together to form trimethylene; R 2 and R 4 may be taken together to form trimethylene; R 5 represents optionally protected hydroxyl, or a group represented by formula: -NR 51 R 52 wherein R 51 and R 52 are the same or different and represent lower alkyl, carboxy lower alkyl, aryl, or aryl lower alkyl, or R 51 and R 52 may be taken together with the nitrogen atom to which they are attached to form a pyrrolidine ring, a piperidine ring, a morpholine ring, or an optionally substituted piperazine ring; R 6 and R 7 are the same or different and represent hydrogen, halogen, or lower alkyl; when R 5 is the group represented by formula, R 51 and R 6 may be taken together to form trimethylene, or R 52 and R 7 may be taken together to form trimethylene; ring A represents an optionally substituted aromatic ring or an optionally substituted heteroaromatic ring; V represents O or SiR 10 R 11 wherein R 10 and R 11 are the same or different and represent hydrogen or lower alkyl; W represents CH 2, CO, or S p; Z represents O or NR 9 wherein R 9 represents hydrogen or alkyl; m and n are the same or different and represent 0 or 1; and p represents 1 or 2, in combination with a compound having the same or different 3 to 5 coordinating positions selected from the group consisting of amino, carboxyl, and pyridine ring, wherein the compound is selected from pyridylmethyl amine compounds and aminocarboxylic acid compounds and salts of these compounds,or in combination with a compound selected from N,N,N',N'-ethylenediaminetetraacetic acid (EDTA), 1,3-propanediamine tetraacetic acid (PDTA), ethylenediamine triacetic acid (HEDTA), 1,2-diaminocyclohexane tetraacetic acid (CyDTA), N,N-bis(2-hydroxybenzyl) ethylenediamine diacetic acid (HBED), ethylenediamine disuccinic acid (EDDS), hexamethylenediamine tetraacetic acid (HDTA), diaminopropane tetraacetic acid (Methyl-EDTA) and salts of these compounds, wherein the compound represented by formula (fluorescence probe) and the compound having coordinating positions are mixed.