A polishing composition contains colloidal silica particles having protrusions on the surfaces thereof. The average of values respectively obtained by dividing the height of a protrusion on the surface of each particle belonging to the part of the colloidal silica particles that has larger particle diameters than the volume average particle diameter of the colloidal silica particles by the width of a base portion of the same protrusion is no less than 0.245. Preferably, the part of the colloidal silica particles that has larger particle diameter than the volume average particle diameter of the colloidal silica particles has an average aspect ratio of no less than 1.15. Preferably, the protrusions on the surfaces of particles belonging to the part of the colloidal silica particles that has larger particle diameters than the volume average particle diameter of the colloidal silica particles have an average height of no less than 3.5 nm.