Disclosed is a method for measuring a clotting time, including: a mixing step of mixing a blood sample, an activator, a phospholipid, and a manganese ion-forming compound to obtain a specimen; and a measurement step of mixing the specimen obtained in the mixing step with a calcium salt to prepare a measurement specimen and measuring the clotting time of the measurement specimen, wherein the blood sample is mixed with the manganese ion-forming compound separately from the activator and the phospholipid in the mixing step.