PURPOSE: To improve measuring accuracy in a fluid speed measuring method to measure fluid speed by analyzing the direction and an interval of a Young's stripe obtained by the optical Fourier transform after a multiple exposure image of particles floated in fluid is recorded by exposure of a pulse-shaped laser beam.

CONSTITUTION: A continuous laser beam from a laser beam source 1 is intermitted in a pulse shape by a chopper 8, and is transformed into a sheet-shaped luminus flux 3 by a cylinder lens 2, and fluid 4 floating particles 5 is exposed plural times, and a multiple exposure image of the particles 5 is photographed on a film 7. In that case, when exposure time and exposure interval time are set in plural-time exposure, the multiple exposure image of locus images of the particles 5 is recorded, and is also made to be recorded in a mutual before and behind locus image nonoverlapping length in plural-time exposure order. Thereby, information in the Young's stripe direction is emphasized, and an analytical error in the Young's stripe direction is reduced, and measuring accuracy can be improved.