To provide a coating agent capable of forming a coating layer or film excellent in moisture permeability, flexibility and tensile strength.A coating agent comprising an isocyanate group-containing prepolymer obtainable by reacting a polyol mixture (A) and a polyisocyanate compound (B), wherein the polyol mixture (A) is a mixture of at least two polyoxyalkylene polyols which are different in the number of hydroxy groups or the hydroxy value, and said mixture has an average number of hydroxy groups being from 2.2 to 2.6, a molecular weight per hydroxy group being from 1,500 to 10,000 and an oxyethylene group content being at least 60 mass %.