A fluorescent nanoparticle probe comprising: a molecular assembly composed of an amphiphilic block polymer having a hydrophilic block chain and a hydrophobic block chain; and a fluorescent dye encapsulated in the molecular assembly, wherein (a) the hydrophilic block chain comprises, as an essential hydrophilic structural unit, a unit selected from a sarcosine unit and an alkylene oxide unit, and has the 20 or more essential hydrophilic structural units, (b) the hydrophobic block chain comprises, as an essential hydrophobic structural unit, a unit selected from the group consisting of an amino acid unit and a hydroxylic acid unit, and has the 15 or more essential hydrophobic structural units, and (c) the fluorescent dye is a cyanine compound represented by the following structural formula: wherein R 1 and R 2 may be the same or different from each other and each is a hydrocarbon group which may be substituted; R 3 is a bivalent hydrocarbon group which may be substituted; X is a halogen, an aryloxy group, or a thioaryloxy group; A - is an anion and m is 0 or 1; and a ring B and a ring D may be the same or different from each other and each is a nitrogen-containing bicyclic or tricyclic aromatic heterocycle, and two or more molecules of the fluorescent dye are encapsulated in a self-quenching state by association in the single molecular assembly, wherein the fluorescent dye is encapsulated in the molecular assembly in an amount of 1 to 50 mol% with respect to a total amount of the amphiphilic block polymer and the fluorescent dye, wherein the fluorescent nanoparticle recovers fluorescence by contact with a blood component. a molecular assembly composed of an amphiphilic block polymer having a hydrophilic block chain and a hydrophobic block chain; and a fluorescent dye encapsulated in the molecular assembly, wherein (a) the hydrophilic block chain comprises, as an essential hydrophilic structural unit, a unit selected from a sarcosine unit and an alkylene oxide unit, and has the 20 or more essential hydrophilic structural units, (b) the hydrophobic block chain comprises, as an essential hydrophobic structural unit, a unit selected from the group consisting of an amino acid unit and a hydroxylic acid unit, and has the 15 or more essential hydrophobic structural units, and (c) the fluorescent dye is a cyanine compound represented by the following structural formula: wherein R 1 and R 2 may be the same or different from each other and each is a hydrocarbon group which may be substituted; R 3 is a bivalent hydrocarbon group which may be substituted; X is a halogen, an aryloxy group, or a thioaryloxy group; A - is an anion and m is 0 or 1; and a ring B and a ring D may be the same or different from each other and each is a nitrogen-containing bicyclic or tricyclic aromatic heterocycle, and two or more molecules of the fluorescent dye are encapsulated in a self-quenching state by association in the single molecular assembly, wherein the fluorescent dye is encapsulated in the molecular assembly in an amount of 1 to 50 mol% with respect to a total amount of the amphiphilic block polymer and the fluorescent dye, wherein the fluorescent nanoparticle recovers fluorescence by contact with a blood component. (a) the hydrophilic block chain comprises, as an essential hydrophilic structural unit, a unit selected from a sarcosine unit and an alkylene oxide unit, and has the 20 or more essential hydrophilic structural units, (b) the hydrophobic block chain comprises, as an essential hydrophobic structural unit, a unit selected from the group consisting of an amino acid unit and a hydroxylic acid unit, and has the 15 or more essential hydrophobic structural units, and (c) the fluorescent dye is a cyanine compound represented by the following structural formula: wherein R 1 and R 2 may be the same or different from each other and each is a hydrocarbon group which may be substituted; R 3 is a bivalent hydrocarbon group which may be substituted; X is a halogen, an aryloxy group, or a thioaryloxy group; A - is an anion and m is 0 or 1; and a ring B and a ring D may be the same or different from each other and each is a nitrogen-containing bicyclic or tricyclic aromatic heterocycle, and two or more molecules of the fluorescent dye are encapsulated in a self-quenching state by association in the single molecular assembly, wherein the fluorescent dye is encapsulated in the molecular assembly in an amount of 1 to 50 mol% with respect to a total amount of the amphiphilic block polymer and the fluorescent dye, wherein the fluorescent nanoparticle recovers fluorescence by contact with a blood component. wherein R 1 and R 2 may be the same or different from each other and each is a hydrocarbon group which may be substituted; R 3 is a bivalent hydrocarbon group which may be substituted; X is a halogen, an aryloxy group, or a thioaryloxy group; A - is an anion and m is 0 or 1; and a ring B and a ring D may be the same or different from each other and each is a nitrogen-containing bicyclic or tricyclic aromatic heterocycle, and two or more molecules of the fluorescent dye are encapsulated in a self-quenching state by association in the single molecular assembly, wherein the fluorescent dye is encapsulated in the molecular assembly in an amount of 1 to 50 mol% with respect to a total amount of the amphiphilic block polymer and the fluorescent dye, wherein the fluorescent nanoparticle recovers fluorescence by contact with a blood component.