A method of producing a polycarbonate or the like which is excellent in hydrolysis and heat stability such as a reduction in molecular weight at the time of molding and color retention and moldability such as releasability and transferability, rarely experiences residence deterioration such as coloring, crosslinking and gel formation and has an extremely small content of foreign matter by employing a special structure for an opening in the side surface of a horizontal cylindrical vacuum resin treating apparatus, supplying a molten polycarbonate into a unidirectional rotary intermeshing double-screw extruder under specific conditions, adding a specific agent under specific conditions and cleaning the inner surface of a production apparatus by a specific method.