A polishing composition for a magnetic disk substrate of the present invention includes water, silica particles, and at least one or more selected from an acid, a salt of the acid, and an oxidizing agent. The silica particles are observed with a transmission electron microscope to measure a maximum diameter and a projected area of each particle, and a value obtained by dividing the area of a circle whose diameter is the maximum diameter of a silica particle by the projected area of the silica particle and multiplying the result by 100, is in the range of 100 to 130.