A polishing composition for use in polishing an object to be polished, which comprises abrasive grains, a dispersing medium, and an additive, wherein the abrasive grains are surface-modified, the additive is represented by the following formula 1: wherein in the formula 1, X1 is O or NR4, X2 is a single bond or NR5, R1 to R5 are each independently a hydrogen atom; a hydroxy group; a nitro group; a nitroso group; a C1-4 alkyl group optionally substituted with a carboxyl group, an amino group, or a hydroxy group; or CONH2; with the proviso that R2 and R5 may form a ring; when X2 is a single bond, R3 is not a hydrogen atom, or R1 to R3 are not a methyl group; and when X2 is NR5 and three of R1 to R3 and R5 are a hydrogen atom, the other one is not a hydrogen atom or a methyl group; and a pH is 5.0 or less. According to the present invention, a polishing composition capable of polishing not only polycrystalline silicon but also a silicon nitride film at high speed and also suppressing a polishing speed of a silicon oxide film is provided.