A composition comprising a silicon-containing copolymer having a number-average molecular weight of 500 to 1,000,000, having SiO bond in the polymer and containing at least the structural units represented by the following general formulae (I) and (II) and, if necessary, one or more of the structural units represented by the following general formulae (III) to (VII) and a cross-linking agent is reacted at −20 to 100° C. for 1 to 3 hours. The resultant reaction composition is coated on a substrate and cured by heating to a temperature of 150° C. or above, for example, 250° C. to obtain a cured product of a silicon-containing copolymer which has a high heat resistance, a high light transmission, a low relative dielectric constant and a high chemical resistance and which has a strong mechanical strength and a good flexibility. wherein R1 to R6, R8 and R9 each independently represents an alkyl group, an alkenyl group, a cycloalkyl group, an aryl group, an aralkyl group, an alkylamino group, an alkylsilyl group or an alkoxy group, R7 represents a divalent group, R10 represents a divalent aromatic group, and A represents NH or O.