A method for manufacturing isoindolic compound, which can adopt a condition capable of being put to industrially practical use, which can produce stable isoindolic compound, and which can further produce the isoindolic compound with a high yield. The method for manufacturing isoindolic compound includes a thermal treatment step where a compound of which molecule includes a structure of pyrrole fused with bicyclo [2.2.2] octadiene skeleton, and includes a formula (I): is subjected to a supercritical carbon dioxide atmosphere. The thermal treatment step is preferably performed at a temperature of not less than 50° C. and not more than 300° C.