[Purpose] To provide a thermosetting composition with which it is possible to obtain paint films which not only display stain resistance for a short period after application, but with which excellent stain resistance can also be expected for longer periods, and which comply with the paint film properties required for painted steel sheet such as water resistance and bending workability, and have high environmental protection and safety qualities. [Solution] The present invention relates to a thermosetting composition, wherein it contains, as essential components, (A) a polyester resin of number average molecular weight 1,000-10,000 and hydroxyl value 5-200 mg KOH/g and, (B) 0.5 to 2.0 equivalents, relative to the hydroxyl groups of the aforesaid component (A), of a compound (B) which is a blocked aliphatic polyisocyanate compound having reactivity to the hydroxyl groups of component (A), wherein the blocks are methyl ethyl ketone oxime and/or ε-caprolactam, (C) 0.05 to 5 mass %, relative to the total resin solids mass of component (A) and component (B), of an organotin compound represented by the following general formula (I) [Chem.1] R1—Sn-(O)C═(O)—R2)3 (I) (R1 in the formula represents an organic group wherein it has 1-24 carbons, and is directly bound to the Sn atom via a C atom. R2 represents an organic group wherein it has 1-24 carbons, and is directly bound to the C atom of the (C═O) via a C atom), and (D) 0.5 to 50 mass %, relative to the total resin solids mass of component (A) and component (B), of an organosilicate represented by the following general formula (II) and/or a condensation product thereof [Chem.2] (R3, R4, R5 and R6 in the formula are each a hydrogen atom or 1-10 carbon organic group, and may be the same or different, n is 1).