A novel nodular silica sol adapted to use as a polishing material for polishing, for instance, CMP.The nodular silica sol has a ratio of an average particle diameter (r) measured by the dynamic light scattering method versus a particle diameter (r′) converted to that of an equivalent sphere computed from an average specific surface area measured by means of the nitrogen absorption method (r/r′, referred to as “association ratio”) in a range from 1.2 to 10, the particle diameter (r′) in a range from 5 to 200 nm, and the specific surface area in a range from 13 to 550 m2/g. The nodular silica particles have heterogeneous forms, and contents of Ca and Mg contained in the nodular silica particles are below 1000 ppm against SiO2 respectively.