PURPOSE: To measure the flow velocity and the concentration of a fluid over a wide range by using a single tracer by a method in which the fluid to which the tracer has been added is irradiated with a beam of flat light, the image of scattered light by the tracer is sensed in two long and short time zones and the flow velocity and the concentration are obtained from respective images.

CONSTITUTION: In order to measure a methane concentration and a flow velocity in a virtual combustion chamber 36, the compressed air from a compressed-air supply source 44 is supplied to the combustion chamber via a carburetor 50 after a tracer has been mixed with methane from a methane-gas cylinder 56 via the carburetor 50. On the other hand, a laser beam 29 from a laser source 28 is changed to a flat laser beam 35 by an optical system 32, and a cylinder 21 is irradiated with the laser beam perpendicularly to its axial line. Then, a beam of scattered light by the tracer is passed through a reflecting mirror 38, a shutter 40 and an image intensifier 41, and the image of a plane figure is sensed by a TV 42 when the tracer is moved. At this time, the image is sensed by dividing the opening and shutting operation of the shutter 40 into two long and short times zones, it is processed 52, the flow velocity of methane is obtained from the image in the long time zone and the methane concentration is obtained from the image in the short time zone.