Process for producing a concentrated solution for a photoresist-stripping liquid having low water content

There is provided a process for producing a concentrated solution of quaternary ammonium hydroxide which is characterized in that quaternary ammonium hydroxide in a form of water-containing crystals or of an aqueous solution is mixed with a water-soluble organic solvent selected from the group consisting of glycol ether, glycol and triol and the resulting mixed solution is subjected to a thin-film distillation in vacuo so as to evaporate the low boiling material. In accordance with this process, a concentrated solution of quaternary ammonium hydroxide having low water content is able to be easily produced.