A nanodiamond according to the present invention has acidic functional groups, contains the acidic functional groups in a number density of 1 or more per square nanometer in the nanodiamond surface, and has a specific surface area of 150 m2/g or more. Particles of the nanodiamond preferably have a D50 (median diameter) of 9 nm or less. The nanodiamond is preferably derived from a nanodiamond synthesized by a detonation technique (in particular, an air-cooling detonation technique).