The present invention provides a novel fluorescent nanoparticle imaging probe having a switching function (a function to quench a fluorescent dye during nanoparticle preparation, and emit fluorescence during imaging). A switching 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, (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 cyanine compound represented by the formula (I):and two or more molecules of the fluorescent dye are encapsulated in the single molecular assembly.
