A nanodiamond particle including an NV center having ODMR intensity enhanced, of which surface is modified with a functional group containing a heteroatom, is provided. This nanodiamond particle as being chemically modified can serve for a fluorescent molecular probe which can be made use of in a biological system. By tracking a rotational motion of the NV center included in this fluorescent molecular probe, structural change of a protein can be analyzed in real time. The functional group containing a heteroatom can be at least any functional group of a hydroxyl group and a hydroxyalkyl group, or a carboxyl group.