A particle image velocimetry system is provided which supplies tracerparticles to aflow field around an object (12) from tracer particle supply means, takes animage of reflectedlight by imaging means (32A, 32B) by irradiating the tracer particles twicewith laser light atdifferent times, and determines a velocity vector of the flow field based onthe two imagesobtained of the tracer particles. The two images are each divided into aplurality of testregions, and when first peak value (fp)/second peak value (sp) = 1.2 issatisfied bycomparison between a first peak value fp and a second peak value sp of a cross-correlationvalue of a luminance pattern of tracer particles in each test region of thetwo images, it isdetermined that the reliability of the velocity vector is high. Thus, it ispossible to enhancethe precision of measurement of the state of flow by reliably determiningerroneous vectors.
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