A detection unit 113 measures scattered light intensity E as a coagulation index. A first curve fitting function 120B-a c calculates, at a predetermined time interval, a parameter for an approximate curve of a coagulation curve by curve fitting, based on the measured scattered light intensity E. An early reaction checking function 120B-b determines whether there is a lag phase as an early coagulation index change, based on the parameter for the approximate curve. The parameter has been calculated at the predetermined time interval by the first curve fitting function 120B-a. A clotting time calculation function 120B-f calculates clotting time, based on whether there is the lag phase.