1. A particle image velocimetry method that involves supplying tracer particles to a flow field around an object ( 12 ) from tracer particle supply means ( 13 ), taking an image of reflected light by imaging means ( 32 A, 32 B) by irradiating the tracer particles twice with laser light at different times, and determining a velocity vector of the flow field based on the two images obtained of the tracer particles, characterized in that the two images are each divided into a plurality of test regions, and the reliability of the velocity vector is determined 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.