A heat- or photo-curable composition comprising: a polysiloxane which is produced by reacting a silicon compound (i) represented by the formula: R1nSi(X)4-n (wherein R1 represents an alkyl group, an aryl group or the like; X represents a chlorine atom or an alkoxy group; and n represents 0 to 2) with a silicon compound (ii) represented by the formula (b) or (c) (wherein R2 to R7 independently represent an alkyl group or the like; M1 and M2 independently represent an arylene group, an alkylene group or the like; and Y1 to Y6 independently represent a chlorine atom or an alkoxy group) in the presence of an alkali catalyst or an acid catalyst; a polymerization initiator which enables the generation of an acid or a base by the action of heat or light; and a solvent. The composition enables the formation of a thick film. When the composition is coated onto a substrate, is then heated or exposed to light, is then developed if necessary, and is then heated and cured at a low temperature, a cured film can be formed.