Abstract: Blood information such as hemolysis (a plasma-free hemoglobin concentration) and a blood coagulation level (thrombus) can be obtained by extracting only reflected light in a plasma layer, and non-invasively and continuously obtaining information only on a plasma component independently of a hematocrit without separating blood components by a mechanical or chemical process. First measurement light 30 is caused to be incident on a boundary surface between blood 10 flowing through a flow cell 40 formed of a transparent material having a different refractive index from plasma (layer) 12 in the blood 10 and the flow cell 40, from an oblique direction at an angle smaller than 90 degrees. Reflected light 32 regularly reflected at the boundary surface between the flow cell 40 and the blood 10 is subjected to spectrometry. Information on a plasma component (a refractive index Np of plasma) is obtained from an absorption spectrum measured.