[Abstract] [Problem] To provide a novel fluorescent nanoparticle imaging probe having a switching function (a function to quench a fluorescent dye in a blood component and emit fluorescence in a tumor or an inflamed site to be imaged).[Solution] 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 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, (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 (c) the fluorescent dye is a polylactic acid-bound cyanine compound comprising: a fluorescent group represented by the formula (I):[Chemicaland a polylactic acid group having 5 to 50 lactic acid units, and two or more molecules of the fluorescent dye are encapsulated in the single molecular assembly. [Selected Figure] Fig.1
