A heavy oil such as an atmospheric pressure residue, a reduced pressure residue of petroleum, etc. is heated to 400.degree. to 500.degree. C. to carry out polycondensation and provide a pitch containing mesophase microspheres. This pitch is once cooled to 200.degree. to 400.degree. C. and a turbulent flow is imparted thereto to cause agglomeration of the mesophase microspheres. The resulting agglomerates are separated to obtain a crystallizable material enriched with quinoline insolubles. Production of the crystallizable material is preferably conducted in a separation tank accommodating the lower part of a heating polycondensation reactor (6) and having a stirring device (12).