A blood analyzer configured to discriminate a cause of a reduction in platelets, the blood analyzer comprising: a measurement unit comprising a first detector including a flow cell and an optical detector configured to detect optical information from the blood cells, a second detector configured to measure the blood cells by a sheath flow direct current detection method, and a third detector configured to measure hemoglobin concentration of a blood sample, the measurement unit being configured to measure the blood sample by detecting at least optical information as measurement data from the blood cells in the blood sample, and to output the measurement data; a controller configured to: generate analysis data on each of the blood cell types including red blood cells, white blood cells, and platelet by analyzing the measurement data outputted from the measurement unit, and generate support information from the analysis data including first analysis data on platelets pertaining to an immature platelet fraction and second analysis data on red blood cells, the immature platelet fraction being the ratio between a number of immature platelets and a number of platelets; and an output unit configured to output the support information; wherein the support information includes first reduction cause information for platelets pertaining to increased destruction of platelets and second reduction cause information for platelets pertaining to decreased production of platelets, and the controller is configured to generate the first cause information and the second cause information; and the first reduction cause information includes an increased destruction score and the second cause information includes a decreased production score. a measurement unit comprising a first detector including a flow cell and an optical detector configured to detect optical information from the blood cells, a second detector configured to measure the blood cells by a sheath flow direct current detection method, and a third detector configured to measure hemoglobin concentration of a blood sample, a first detector including a flow cell and an optical detector configured to detect optical information from the blood cells, a second detector configured to measure the blood cells by a sheath flow direct current detection method, and a third detector configured to measure hemoglobin concentration of a blood sample, the measurement unit being configured to measure the blood sample by detecting at least optical information as measurement data from the blood cells in the blood sample, and to output the measurement data; a controller configured to: generate analysis data on each of the blood cell types including red blood cells, white blood cells, and platelet by analyzing the measurement data outputted from the measurement unit, and generate support information from the analysis data including first analysis data on platelets pertaining to an immature platelet fraction and second analysis data on red blood cells, the immature platelet fraction being the ratio between a number of immature platelets and a number of platelets; and generate analysis data on each of the blood cell types including red blood cells, white blood cells, and platelet by analyzing the measurement data outputted from the measurement unit, and generate support information from the analysis data including first analysis data on platelets pertaining to an immature platelet fraction and second analysis data on red blood cells, the immature platelet fraction being the ratio between a number of immature platelets and a number of platelets; and an output unit configured to output the support information; wherein the support information includes first reduction cause information for platelets pertaining to increased destruction of platelets and second reduction cause information for platelets pertaining to decreased production of platelets, and the controller is configured to generate the first cause information and the second cause information; and the first reduction cause information includes an increased destruction score and the second cause information includes a decreased production score.