To obtain a coating agent enabling a coating operation to be carried out readily and speedily, and capable of providing a coated membrane excellent in durability or the like by including an aminosilane-based hardening agent and a specific alkoxysilane-modified random copolymer in a volatile solvent as essential ingredients.

This coating agent contains an aminosilane-based hardening agent in combination with a methacrylic-acrylic type alkoxysilane-modified random copolymer of formula I (R1 is a group of formula II; R2 is a group of formula III, m is 50-100; n is 20-50) as essential ingredients. The aminosilane- based hardening agent is preferably the one consisting essentially of tetraalkoxysilane of formula IV (R3 is CH3, C2H5 or the like). The coating agent preferably further contains a urethane resin-based pigment as a coloring matter. The formulated amount of the random copolymer is preferably 0.01-20 wt.% As a result, the objective coating material capable of maintaining the properties over a long period can be provided.