Curable composition, cured product and method for forming insulating film

A curable composition comprising a modified maleimide compound (A), a radical generator (C), a thiol compound (D), and a protic acid (E), wherein

• the modified maleimide compound (A) has one or more groups represented by the following formula (a1):
• and,
• the group represented by the formula (a1) is bonded to a carbon atom in an aliphatic hydrocarbon group or an aromatic group in the modified maleimide compound (A), wherein Ra01 and Ra02 are each independently a hydrogen atom, an alkyl group having 1 or more and 6 or less carbon atoms, a cycloalkyl group having 3 or more and 8 or less carbon atoms, or an aryl group having 6 or more and 12 or less carbon atoms, and wherein
• the protic acid (E) is selected from the group consisting of formic acid, propionic acid, butyric acid, valeric acid, isovaleric acid, oxalic acid, phthalic acid, benzoic acid, salicylic acid, 4-hydroxybenzoic acid, lactic acid, citric acid, tartaric acid, malonic acid, malic acid, succinic acid, glycolic acid, glutamic acid, aspartic acid, maleic acid, caproic acid, caprylic acid, myristic acid, stearic acid, palmitic acid, pyruvic acid, ascorbic acid, adipic acid, phenol, 1-naphthol, 2-naphtholmethanesulfonic acid, methylphosphonic acid, ethylphosphonic acid, and phenylphosphonic acid.