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 polylactic acid-bound cyanine compound comprising: a fluorescent group represented by the following structural formula: wherein R 1 is a hydrocarbon group which may be substituted, and R 2 is a bivalent hydrocarbon group which may be substituted; A - is an anion and m is 0 or 1; a ring B and a ring D may be the same or different from each other and each is a nitrogen-containing heterocycle; and L is a linking group that constitutes a polymethine chain, which may include a ring structure, and which may be substituted; and a polylactic acid group having 5 to 50 lactic acid units, wherein molecules of the fluorescent dye are encapsulated in a self-quenching state by association in the single molecular assembly in an amount of 20 to 50 mol% with respect to a total amount of the amphiphilic block polymer and the fluorescent dye so that the fluorescent nanoparticle remains in a self-quenching state 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 polylactic acid-bound cyanine compound comprising: a fluorescent group represented by the following structural formula: wherein R 1 is a hydrocarbon group which may be substituted, and R 2 is a bivalent hydrocarbon group which may be substituted; A - is an anion and m is 0 or 1; a ring B and a ring D may be the same or different from each other and each is a nitrogen-containing heterocycle; and L is a linking group that constitutes a polymethine chain, which may include a ring structure, and which may be substituted; and a polylactic acid group having 5 to 50 lactic acid units, wherein molecules of the fluorescent dye are encapsulated in a self-quenching state by association in the single molecular assembly in an amount of 20 to 50 mol% with respect to a total amount of the amphiphilic block polymer and the fluorescent dye so that the fluorescent nanoparticle remains in a self-quenching state 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 polylactic acid-bound cyanine compound comprising: a fluorescent group represented by the following structural formula: wherein R 1 is a hydrocarbon group which may be substituted, and R 2 is a bivalent hydrocarbon group which may be substituted; A - is an anion and m is 0 or 1; a ring B and a ring D may be the same or different from each other and each is a nitrogen-containing heterocycle; and L is a linking group that constitutes a polymethine chain, which may include a ring structure, and which may be substituted; and a polylactic acid group having 5 to 50 lactic acid units, wherein molecules of the fluorescent dye are encapsulated in a self-quenching state by association in the single molecular assembly in an amount of 20 to 50 mol% with respect to a total amount of the amphiphilic block polymer and the fluorescent dye so that the fluorescent nanoparticle remains in a self-quenching state by contact with a blood component. a fluorescent group represented by the following structural formula: wherein R 1 is a hydrocarbon group which may be substituted, and R 2 is a bivalent hydrocarbon group which may be substituted; A - is an anion and m is 0 or 1; a ring B and a ring D may be the same or different from each other and each is a nitrogen-containing heterocycle; and L is a linking group that constitutes a polymethine chain, which may include a ring structure, and which may be substituted; and a polylactic acid group having 5 to 50 lactic acid units, wherein molecules of the fluorescent dye are encapsulated in a self-quenching state by association in the single molecular assembly in an amount of 20 to 50 mol% with respect to a total amount of the amphiphilic block polymer and the fluorescent dye so that the fluorescent nanoparticle remains in a self-quenching state by contact with a blood component.