This invention is drawn to a curable composition that has excellent smoothness when a plastic substrate is coated therewith. Also disclosed are active energy beam curable compositions, such as actinically curable compositions, that can be cured without using a photopolymerization initiator, or with the use of a small amount thereof. This object is achieved by the presence of surface treated inorganic oxide microparticles. Such particles are obtained by reacting an alkoxysilane compound (a1) represented by the Formula (1) below and inorganic oxide microparticles (a2) in the presence of water and an organic solvent: (P—SiO3/2)1-a(P—Si(R0)O2/2)a(O1/2R3)z (1) in which P denotes a maleimide containing group, R3 denotes a hydrogen atom or a monovalent organic group, R0 denotes a monovalent organic group, a is 0 or a positive number and satisfies 0≦a