In the production of an N-unsubstituted maleinimide by the ring-closing alcohol-removing treatment of an N-unsubstituted maleinamic ester for conversion into an imide, an acid catalyst or a mono(cyclo)alkyl sulfuric ester is used for the reaction. In this case, the alcohol concentration in the reaction solution is preferable to be adjusted to a low level. By using a maleinamic acid and an alcohol in conjunction with an inert solvent and an acid catalyst, a mixture of maleinamic acid with an acid catalyst, or a mono(cyclo)alkyl sulfuric ester, a corresponding maleinamic ester is obtained. In a method for the production of a maleinamic acid by the reaction of maleic anhydride with ammonia in an inert solvent, the maleinamic acid is obtained by placing a solution of the maleic anhydride in the inorganic solvent in a reactor, meanwhile introducing ammonia gas into the empty space part of the reactor, and allowing the ammonia gas to be absorbed in the solution through the surface of contact between the solution and the ammonia gas. Further, in the method for the production of a maleinamic acid by the reaction of maleic anhydride with ammonia in an inert solvent, the reaction is carried out while the concentration of the maleic anhydride in the reaction solution is kept at a level of not more than 10% by weight or the ammonia concentration in the reaction solution is kept at a level of not less than 0.001% by weight.