A silica-based composite fine particle dispersion, wherein the silica-based composite fine particles comprise child particles primarily composed of crystalline ceria, wherein the content of crystalline ceria is not less than 90 % by mass based on the child particles, the child particles being formed on surfaces of mother particles, the mother particles having a content of amorphous silica of not less than 90 mass%, the silica-based composite fine particle dispersion comprising silica-based composite fine particles having an average particle size of 50 to 350 nm measured by dynamic light scattering or laser diffraction scattering, and having characteristic features of to : the silica-based composite fine particles have a mass ratio between silica and ceria of 100:11 to 316 determined by ICP plasma emission spectroscopy; only a ceria crystal phase is detected when the silica-based composite fine particles are subjected to X-ray diffractometry; a crystallite size in the crystalline ceria, as measured by subjecting the silica-based composite fine particles to X-ray diffractometry, is 10 to 25 nm, in the silica-based composite fine particles, a percentage of a number of particles having a ratio between the shorter diameter and the longer diameter of not more than 0.8, is not more than 35%, wherein the longer and shorter diameter are determined by an image analysis method as follows: in a projection image obtained via transmission electron spectroscopy, the particles have a maximum diameter deemed as major axis and defined as longer diameter, and when a point dividing the major axid length on the major axis into two halves is determined, two points of intersection of a line orthogonal to the major axis at the point with the periphery of the particle are determined, and the distance between the two points is measured and defined as shorter diameter , and each of the silica-based composite fine particles has a silica coating formed partly on the surfaces of the child particles. the silica-based composite fine particles have a mass ratio between silica and ceria of 100:11 to 316 determined by ICP plasma emission spectroscopy; only a ceria crystal phase is detected when the silica-based composite fine particles are subjected to X-ray diffractometry; a crystallite size in the crystalline ceria, as measured by subjecting the silica-based composite fine particles to X-ray diffractometry, is 10 to 25 nm, in the silica-based composite fine particles, a percentage of a number of particles having a ratio between the shorter diameter and the longer diameter of not more than 0.8, is not more than 35%, wherein the longer and shorter diameter are determined by an image analysis method as follows: in a projection image obtained via transmission electron spectroscopy, the particles have a maximum diameter deemed as major axis and defined as longer diameter, and when a point dividing the major axid length on the major axis into two halves is determined, two points of intersection of a line orthogonal to the major axis at the point with the periphery of the particle are determined, and the distance between the two points is measured and defined as shorter diameter , and each of the silica-based composite fine particles has a silica coating formed partly on the surfaces of the child particles. <EOS>