A polishing composition contains: silicon dioxide having an average primary particle diameter of 40 nm or more as calculated from the specific surface area determined by the BET method; a nitrogen-containing water-soluble polymer; and a basic compound. The value of B/A is 1 or more and less than 7,000 and the value of C/A is 5,000 or more and less than 1,500,000 when in one liter of the polishing composition, A is defined as the number of silicon dioxide, B is defined as the number of monomer units of the nitrogen-containing water-soluble polymer, and C is defined as the number of molecules of the basic compound. Alternatively, the value of B/A is 1 or more and less than 7,000 and the value of C/A is 5,000 or more and less than 100,000. The polishing composition is used, for example, for polishing a semiconductor substrate.