A coating agent composition of the present invention consists of: (A) an acrylic-type copolymer that has a weight-average molecular weight within the range of 3,000 to 100,000 and contains in its side molecular chains hydrolysable silyl groups of the following formula: —COO-Q-SiR2n(OR3)3-n and organosiloxysilyl groups of the following formula: —COO-Q-Si(OSiR43)3; and (B) a condensation-reaction accelerating catalyst. Prior to curing, the composition demonstrates excellent storage stability and coating properties, while after curing the composition produces a coating film of high hardness that has excellent water-repellent properties and durability.