PURPOSE: To easily measure the speed and the direction of a complicated flow by comparing plural scattered light images obtained at the time when tracer particles moved at the same speed of the fluid pass sheet light.

CONSTITUTION: The shake speed of cameras 106a and 106b is designated by a controller 101 and an exposure start command is issued. A shutter 103 is opened to generate sheet light 105 in the flow passage after the start of shake, and cameras 106a and 106b photograph loca of tracer particles 107 moved at he same speed as the fluid while shaking. After a certain time, the controller 102 closes the shutter 103 and indicates the end of exposure to cameras 106a and 106b. Two pictures taken by two cameras 106a and 106b are converted to digital signals by the controller 101 and are subjected to comparing operation to obtain the speed and the direction of the flow on the sheet light. Thus, the speed and the direction of the running fluid are easily discriminated though the flow includes complicated vortexes.