PURPOSE: To prevent drop in S/N ratio when a frequency deviation is detected by a method wherein an error in the detection speed of an object to be measured due to returning beam is suppressed to be small and the quantity of light of the returning beam is made large. ;CONSTITUTION: A laser beam is cast on an object 30 to be measured, and the speed of the object 30 to be measured is measured by the deviation of a beat frequency obtained from a heterodyne interference by two light beams, received scattered light beam or reflected light beam and reference light beam. The laser beam which is cast on the object 30 to be measured is a light beam 24 whose vertical beam diameter is different from the transverse beam diameter when a spatial-light intensity distribution is observed in a cross section perpendicular to an optical axis (Z-axis). As a result, cylindrical lenses 14, 16 and a deflecting prism are used. The speedometer is constituted in such a way that a plane in which a straight line in the optical-axis direction of the laser beam and a straight line in the motion direction of the object 30 to be measured exist, is assumed and that the spatial-light intensity distribution in which the major axis of the cross section of the laser beam which has been made oval-shaped, is made parallel with the normal-line direction of the plane and in which its minor axis exist in the plane, is formed.;COPYRIGHT: (C)1995,JPO
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