A polyimidosiloxane material useful for forming, on a flexible substrate material, a flexible protective coating layer with a high heat resistance, electrical insulating property and curl-resistance, comprises a polymerization-imidization product of an aromatic tetracarboxylic acid component comprising, as a principal ingredient, 2,3,3',4'-biphenyltetracarboxylic dianhydride with a diamine component comprising, as a principal ingredient, a diaminopolysiloxane compound of the formula (I) ##STR1## wherein R.sub.1 is a divalent hydrocarbon residue, R.sub.2 is an C.sub.1-3 alkyl or phenyl group, n is an integer of 3 to 30, and is preferably used, optionally together with an epoxy resin, in the form of a solution or dispersion in an organic solvent to coat a flexible substrate material, for example, a flexible circuit element, therewith.