A composite sol containing colloidal composite particles having a particle diameter measured by dynamic light scattering method of 20 to 500 nm, composed of colloidal silica particles having a specific surface area diameter of 3 to 100 nm and aluminum phosphate bonding the colloidal silica particles or coating and bonding the colloidal silica particles; a process for producing the composite sol; a coating composition for ink receiving layer containing the composite sol; and an ink jet recording medium having an ink receiving layer containing the composite sol.