A novel stable silica sol which does not gel even at low temperatures is prepared by adding a di -or polyhydric alcohol having three or more carbon atoms and a water-soluble melamine resin to a silica sol. A novel water-based coating composition which requires only an applying step, not a baking step nor a drying step, in the use is prepared by adding an aggregate or filler and usual additives for coating material to the aforesaid stable silica sol as binder. By adding further a small amount of a synthetic resin emulsion to said binder, an inorganic coating composition containing a smaller amount of organic materials than that in prior products can be obtained. The coating therefrom is excellent in strength and its fire resistance is not lowered.