Provided is a polymer brush-type filler agent having a high polymer chain density and a wide molecular weight distribution of the polymer chains. Specifically provided are fine particles for chromatography having grafted polymer chains formed by grafting polymer chains via polymerization initiator groups on the surfaces of solid particles, wherein the grafted polymer chains contain specific repetition units, have a specific number average molecular weight and a specific molecular weight distribution, and are formed with a high density of 0.03-0.70 chains/nm<2>.