1. A particle image velocimetry method comprising supplying tracer particles to a flow field around an object from tracer particle supply means, taking an image of reflected light by imaging means by irradiating the tracer particles twice with laser light at different times and different positions of the imaging means relative to the flow field, and determining a velocity vector of the flow field based on the two images obtained of the tracer particles, wherein 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, and wherein when the first peak value (fp)/the second peak value (sp)≧1.2 is satisfied, it is determined that the reliability of the velocity vector is high.