A method for producing a curing agent having an acidic substituent and an unsaturated maleimido group, comprising reacting, in an organic solvent, a maleimide compound (a) having at least two N-substituted maleimido groups in a molecule thereof with an amine compound (b) having an acidic substituent represented by the following formula (I):
- (wherein R1, which is an acidic substituent, represents a hydroxyl group, a carboxyl group, or a sulfonic acid group;
- R2 represents a hydrogen atom, a C1-C5 aliphatic hydrocarbon group, or a halogen atom;
- x is an integer from 1 to 5;
- y is an integer from 0 to 4;
- and the sum of x and y is 5), and
- wherein said organic solvent is selected from the group consisting of cyclohexanone, propylene glycol monomethyl ether and methyl cellosolve.