An objective of the present invention is to provide methods of producing a cyclic organic compound using a continuous stirred tank reactor(s) (CSTR), the methods being capable of achieving excellent impurity-suppressing effects (quality improvement), reduction in reaction-tank size, continuous production, and such. The present inventors conducted studies on cyclization reactions using a CSTR(s), which had not been conventionally used for cyclization reactions for cyclic compounds. As a result, the inventors have found that the present methods can achieve excellent impurity-suppressing effects (quality improvement), reduction in reaction-tank size, continuous production, and such, as compared with conventional cyclization methods. Furthermore, the present inventors have also found that the above-mentioned improvement effects can efficiently be achieved even in the production of cyclic peptides and heterocyclic compounds by applying simulation methods that had been conventionally used mainly at the fine chemicals plant level to the cyclization reactions of the present invention, thereby experimentally predicting the reaction rate of a cyclization reaction, and setting the flow volume (residence time), the concentrations of precursor and cyclic organic compound, and the temperature for the cyclization reaction and such which affect these conditions, in the cyclization reaction using a CSTR(s).