A blood coagulation time measuring method comprising irradiating a mixture of blood plasma and reagent with a beam of specified amount of light, detecting changes in the amount of light scattered by the mixture, converting the resulting scattered light output signals to digital form at specified time interval, feeding the digital data to a computer, computing the difference between each input value and the input value immediately adjacent thereto with respect to time and storing the results, and determining the time, prior to the time at which a maximum difference value is obtained, at which a computed difference value corresponding to 1/n of the maximum difference value is obtained. The time thus determined is the end point of the coagulation time. A system for practicing this method is also disclosed.