A particle image velocimetry system is provided which supplies tracer particles to a flow field around an object (12) from tracer particle supply means, takes an image of reflected light by imaging means (32A, 32B) by irradiating the tracer particles twice with laser light at different times, and determines a velocity vector of the flow field based on the two images obtained of the tracer particles. The two images are each divided into a plurality of test regions, and when first peak value (fp)/second peak value (sp)≧1.2 is satisfied by comparison between a first peak value fp and a second peak value sp of a cross-correlation value of a luminance pattern of tracer particles in each test region of the two images, it is determined that the reliability of the velocity vector is high. Thus, it is possible to enhance the precision of measurement of the state of flow by reliably determining erroneous vectors.