PURPOSE: To accurately measure the flow velocity of a fluid by a simple constitution by a method wherein the image of a beam of scattered light by a tracer flowing in the intermediate layer having a structure of three layers in which the intensity of adjacent layers of light is different from each other and which have been brought into close contact is sensed and the flow velocity is found on the basis of the length of the track of the tracer.

CONSTITUTION: In order to supply a gas to a sucking tube 24 and to measure the flow velocity of the gas inside a combustion chamber 36, a tracer is mixed with the compressed air from a compressed-air supply source 44 and supplied to the combustion chamber 36. On the other hand, a laser beam 29 from an argon ion laser source 28 is changed to a flat laser beam 33 by an optical system 32, it is passed through a filter 34, and it is changed to a laser beam 33 having a structure of three layers whose intensity is different from each other and which have been brought into close contact in the thickness direction. A cylinder 21 is irradiated perpendicularly with the laser beam 35, the image of a beam of scattered light by the tracer is sensed by a TV 42 via a reflecting mirror 38, a shutter 40 and an image intensifier 41, and the image is processed by a processing circuit 52. At this time, since the tracer which has been moved only inside the intermediate layer of the laser beam 35 can be discriminated, the flow velocity can be measured accurately on the basis of its track length and its exposure time.