A blood coagulation analyzer comprising: a sample preparation section (30, 60) for preparing a measurement sample from a blood sample and a predetermined reagent; a detecting section including a light source for emitting light to the measurement sample prepared by the sample preparation section and a light-receiving section for receiving the light from the measurement sample; a first control means for controlling the sample preparation section and the detecting section so as to prepare a first measurement sample from a blood sample and a prothrombin time measurement reagent, a partial thromboplastin time measurement reagent, or an activated partial thromboplastin time measurement reagent and to detect light from the first measurement sample; a derived fibrinogen concentration acquisition means for acquiring a derived fibrinogen concentration using a correlation between the light detected from the first measurement sample by the detecting section and a fibrinogen concentration of a standard sample; a preparation condition changing means for changing a preparation condition of a second measurement sample prepared from the blood sample and a fibrinogen concentration measurement reagent depending on the derived fibrinogen concentration; a second control means for controlling the sample preparation section and the detecting section so as to prepare, based on the preparation condition changed by the preparation condition changing means, the second measurement sample from the blood sample and a fibrinogen concentration measurement reagent and to detect light from the second measurement sample; and a fibrinogen concentration acquisition means for acquiring, based on the light detected by the detecting section from the second measurement sample, a fibrinogen concentration in the blood sample. a sample preparation section (30, 60) for preparing a measurement sample from a blood sample and a predetermined reagent; a detecting section including a light source for emitting light to the measurement sample prepared by the sample preparation section and a light-receiving section for receiving the light from the measurement sample; a first control means for controlling the sample preparation section and the detecting section so as to prepare a first measurement sample from a blood sample and a prothrombin time measurement reagent, a partial thromboplastin time measurement reagent, or an activated partial thromboplastin time measurement reagent and to detect light from the first measurement sample; a derived fibrinogen concentration acquisition means for acquiring a derived fibrinogen concentration using a correlation between the light detected from the first measurement sample by the detecting section and a fibrinogen concentration of a standard sample; a preparation condition changing means for changing a preparation condition of a second measurement sample prepared from the blood sample and a fibrinogen concentration measurement reagent depending on the derived fibrinogen concentration; a second control means for controlling the sample preparation section and the detecting section so as to prepare, based on the preparation condition changed by the preparation condition changing means, the second measurement sample from the blood sample and a fibrinogen concentration measurement reagent and to detect light from the second measurement sample; and a fibrinogen concentration acquisition means for acquiring, based on the light detected by the detecting section from the second measurement sample, a fibrinogen concentration in the blood sample.