PURPOSE: To obtain a coating composition useful for rustproofing coating for metallic products such as automobiles, giving adequate corrosionproofness and durability even in case the resultant film is thin by incorporating a cationic electrodeposition coating with a specific alkylsilane compound.

CONSTITUTION: This coating composition can be obtained by incorporating (A) a cationic electrodeposition coating with (B) an alkylsilane compound of the formula [(l) is 1-20; (m) is 0 or 1; (n) is 1-3; X is H, HS, etc.; R1 is CH2; R2 is C2q+1 ((q) is 1-8) or COCH2] (e.g. 3-glycidoxypropyltrimethoxysilane, 3- acryloxypropyltrimethoxysilane, etc.) so as to be 0.05-5wt.% in alkylsilane content on the whole solid basis. When the compound of the formula is soluble to water, the composition is preferably prepared by the following process: this compound is added to water incorporated with an acid for neutralizing the cationic water-based resin as binder followed by agitation and mixing and then incorporating the resultant mixture with the additional cationic water-based resin to carry out aqueous dispersion.