Provided are: a phosphor-integrated nanoparticle labeling agent which is capable of yielding a sufficient signal intensity even when its final concentration in an immunofluorescent staining reaction system is low (e.g., 0.02 nM) and enables to obtain an immunofluorescently stained image with reduced noise by inhibiting non-specific adsorption of a probe biological substance and a label to substances other than a detection subject; and an immunostaining method using the same. The phosphor-integrated nanoparticle labeling agent is a set which includes: a probe biological substance 3, which is linked to a first binding group substance A via a first hydrophilic polymer-derived spacer 1 having a length of 30 Å to 1,000 Å and specifically binds to a biomolecule 2; and a phosphor-integrated nanoparticle 5, which has a second binding group substance B capable of specifically binding to the first binding group substance A.
