PURPOSE: To safely obtain a high-quality melt product by subjecting an aromatic compound to alkali fusion by a common reactor industrially and economically without using a specific device while improving fluidity of melt and reducing an amount of alkali used.

CONSTITUTION: In subjecting an aromatic compound such as aromatic sulfonic acid, aromatic imide or aromatic halide to alkali fusion, a high-boiling oil which comprises components in coal tar having boiling points of 200-400°C or a fraction consisting essentially of the components, shows approximately at normal temperature and does not exhibit acidity, preferably a polycyclic aromatic compound, especially an alkyl-substituted naphthalene or nitrogen-containing polycyclic aromatic compound such as methylnaphthalene, quinoline, isoquinoline, quinaldine or a fraction consisting essentially of at least one of them is added to the reaction system to improve fluidity of the reaction product. Being stable, the high- boiling oil has no bad influence on the reaction and is reusable. Further this method is relatively readily continued.