A surface-modified nanodiamond includes a base nanodiamond, and at least one polyglycerol-chain-containing group present on at least a surface portion of the base nanodiamond, in which the polyglycerol-chain-containing group is represented by following Formula (1): —X—R   (1) wherein X represents single bond, —NH—, —O—, —COO—, —PH(═O)O—, or —S—; and R represents a polyglyceryl group. X may be single bond or —NH—. The surface-modified nanodiamond is highly soluble or dispersible satisfactorily stably in water and/or polar organic solvents.