Provided is a method of measuring blood coagulation time, the method being capable of LA detection easily and with high sensitivity as compared with the method recommended by the ISTH, without being affected by deficiency of blood coagulation factors even in a blood sample of a warfarin taker, a person who suffers from vitamin K deficiency, or a hepatic failure patient. Disclosed is a method of measuring the blood coagulation time to detect lupus anticoagulant, the method including adding a buffer solution composition containing blood coagulation factors to a blood sample before measurement or at the time of measurement of the blood coagulation time, and measuring the blood coagulation time.