The flow velocity measuring device has a light oscillation part, a metal thin film for causing surface plasmon resonance caused by light emitted from the light oscillation part, and a metal thin film fixed on the metal thin film. A condensing part that converts the incident light into an angle and collects it at a linear focal line position, an antibody immobilization area where the antibody is immobilized on the metal thin film at a position overlapping the focal line position, and a reference area where the antibody is not immobilized Measurement units arranged alternately, a light receiving unit that receives reflected light at the focal line position of the emitted light due to surface plasmon resonance generated at the focal line position for each angle of a plurality of incident light, an antibody fixing region in the measurement unit, and A SPR angle calculation unit that calculates a temporal change in the SPR angle for each reference region; a flow rate calculation unit that calculates a flow rate of the specimen flowing in the flow cell based on the temporal change in the SPR angle obtained by the SPR angle calculation unit; A.
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