Comparison is made between the first and second peak values of cross-correlation values for tracer particle brightness patterns in two images, thereby determining the reliability of a velocity vector. Three velocity components in a plurality of planes located away from one another in a direction perpendicular to a laser sheet are obtained and stacked up on the assumption that the pertinent flow is a steady flow, thereby measuring the velocity field for the three velocity components in a three-dimensional space. An object has a fluorescent surface which changes and reflects the wavelength of a laser beam that goes thereinto. An imaging means takes images of reflected light from tracer particles via a filter which selectively causes a specified wavelength to pass. A tracer particle generation device is provided with a collision jet type nozzle, a compressed air supply source, an oil tank, a first air passage, an oil passage, and a second air passage.
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