PURPOSE: To provide the titled composition containing a specific blocked polyisocyanate and an active hydrogen-containing prepolymer, having low viscosity, high solid content, low heating loss and improved heat resistance, and giving a sufficiently thick coating film with reduced number of times of repeated coatings.

CONSTITUTION: The objective composition is prepared by mixing (A) a blocked polyisocyanate obtained by blocking (i) a polyisocyanate containing carbadiimide group (e.g. 2,4-tolylene diisocyanate) and/or a polyisocyanate containing carbadiimide group and reacted with an active hydrogen compound (preferably a phenolic compound) with (ii) a monovalent active hydrogen compound and (B) a prepolymer containing active hydrogen (e.g. a polymer having two or more active hydrogen atoms reactive with isocyanate group, e.g. polyester, alkyd resin, etc. derived from a polybasic acid and a polyhydric alcohol). The equivalent ratio of the blocked isocyanate group to active hydrogen is 0.6W1.4 to 1.