Abstract: An aspect of the present invention relates to a method of determining a manufacturing condition of a polythiourethane eyeglass lens obtained via a process of obtaining polythiourethane by polymerization reaction of an iso(thio)cyanate compound and a thiol compound, comprising determining a candidate composition for a starting material mixture and a candidate polymerization reaction condition employed in the polymerization reaction in actual manufacturing; polymerizing the starting material mixture of the candidate composition that has been determined under the candidate polymerization reaction condition to obtain a test sample; testing the obtained test sample by irradiation with a xenon lamp to determine whether or not change into red-like color is present following irradiation with the xenon lamp; and determining a starting material mixture composition and a polymerization reaction condition to be employed in the polymerization reaction in actual manufacturing with the use of a result of the determination a