An APTT prolongation factor estimation system includes one or more facilities each including an analyzer which measures a blood coagulation reaction of a subject blood specimen, a database which stores data on a blood coagulation reaction and APTT prolongation factor of blood specimens, and a computer which estimates an APTT prolongation factor of the subject blood specimen based on the data on the blood coagulation reaction from the analyzer and the data stored in the database. The facilities each further includes a data transmission unit which transmits, to the computer, the data on the blood coagulation reaction of the subject blood specimen obtained by the analyzer, and a data reception unit which receives a result of the estimation of the APTT prolongation factor, which is obtained by the computer, regarding the subject blood specimen measured by each of the analyzers.