[Problem] To provide a coating composition capable of forming a hard coating on the surfaces of an optical base member such as a plastic lens featuring highly closely adhering property, excellent transparency and excellent scratch resistance.[Means for Solution] The coating composition comprise (a) a fine granular silica having a grain size of not larger than 30 nm; (b) organosilicon compound components containing an epoxy group-containing organosilicon compound (b1) and tetraalkoxysilane (b2) at a mass ratio (b1/b2) of 0.25 to 4.0; (c) water; and (d) a curing catalyst comprising a complex containing aluminum as a core metal, wherein the fine granular silica (a) is contained in an amount of 15 to 55 parts by mass, the water (c) is contained in an amount of 30 to 200 parts by mass and the curing catalyst (d) is contained in an amount of 0.1 to 5.0 parts by mass per the total amount of the fine granular silica (a) and the organosilicon compound components (b) of 100 parts by mass.