A method for acquiring information on a cause of prolongation of coagulation time, the method comprising: a step of measuring first coagulation time which is coagulation time of a blood specimen of a subject, second coagulation time which is coagulation time of a normal blood specimen, and third coagulation time which is coagulation time of a mixed specimen in which the blood specimen of a subject and the normal blood specimen are mixed, using a coagulation time measuring reagent; a step of measuring fourth coagulation time which is coagulation time of the blood specimen of a subject incubated under predetermined conditions, fifth coagulation time which is coagulation time of the normal blood specimen incubated under the predetermined conditions, and sixth coagulation time which is coagulation time of the mixed specimen incubated under the predetermined conditions, using the reagent; a step of acquiring a first quantification index based on the first, second and third coagulation times, and acquiring a second quantification index based on the fourth, fifth and sixth coagulation times; and a step of executing calculation using the value of the first quantification index and the value of the second quantification index, and acquiring the calculation result as the information on a cause of prolongation of coagulation time, wherein the predetermined conditions are incubation at 15°C or more and 40°C or less for 45 minutes or more and 4 hours or less, wherein the first quantification index and the second quantification index are any one selected from Index of Circulating Anticoagulant, Percent Correction and Response Curve-Score (RC-S), respectively and wherein the information on a cause of prolongation of coagulation time is the result of calculating the value of ratio or difference between the value of the first quantification index and the value of the second quantification index, or a value obtained by combining the value of ratio and the value of difference. a step of measuring first coagulation time which is coagulation time of a blood specimen of a subject, second coagulation time which is coagulation time of a normal blood specimen, and third coagulation time which is coagulation time of a mixed specimen in which the blood specimen of a subject and the normal blood specimen are mixed, using a coagulation time measuring reagent; a step of measuring fourth coagulation time which is coagulation time of the blood specimen of a subject incubated under predetermined conditions, fifth coagulation time which is coagulation time of the normal blood specimen incubated under the predetermined conditions, and sixth coagulation time which is coagulation time of the mixed specimen incubated under the predetermined conditions, using the reagent; a step of acquiring a first quantification index based on the first, second and third coagulation times, and acquiring a second quantification index based on the fourth, fifth and sixth coagulation times; and a step of executing calculation using the value of the first quantification index and the value of the second quantification index, and acquiring the calculation result as the information on a cause of prolongation of coagulation time, wherein the predetermined conditions are incubation at 15°C or more and 40°C or less for 45 minutes or more and 4 hours or less, wherein the first quantification index and the second quantification index are any one selected from Index of Circulating Anticoagulant, Percent Correction and Response Curve-Score (RC-S), respectively and wherein the information on a cause of prolongation of coagulation time is the result of calculating the value of ratio or difference between the value of the first quantification index and the value of the second quantification index, or a value obtained by combining the value of ratio and the value of difference.