Abstract: A thermosetting solution composition composed of a biphenyltetracarboxylic acid compound containing a partial lower aliphatic alkyl ester of 2,3,3?,4?-biphenyltetracarboxylic acid and/or a partial lower aliphatic alkyl ester of 2,2?,3,3?-biphenyltetracarboxylic acid, an aromatic diamine compound in a molar amount larger than a molar amount of the biphenyltetracarboxylic acid compound, a partial lower aliphatic alkyl ester of 4-(2-phenylethynyl)phthalic acid compound in a molar amount as much as 1.8-2.2 times a molar amount corresponding to a difference between the molar amount of the aromatic diamine compound and the molar amount of the biphenyltetracarboxylic acid compound, and an organic solvent composed of a lower aliphatic alcohol is of value for manufacture of a prepreg.