To provide a dynamic shape-measuring device and a method capable of making the measurement accuracy of a polygon mirror stabilized.

A threshold is provided in a peak wavenumber of an interference fringe recorded, corresponding to measurement accuracy required for measurement, and it is determined by a computer 10 whether the peak wavenumber is below the threshold, and thereby only the interference fringe having the peak wavenumber below the threshold is used for measurement. This device is provided with an optical system for irradiating the polygon mirror 1 with light from a semiconductor laser 2; an optical system for generating the interference fringe between reflected light from the polygon mirror 1 and reference light; a CCD camera 7 for imaging an interference fringe image of the polygon mirror 1, the computer 10 for detecting the peak wavenumber of the interference fringe in the interference fringe image, a computing unit 17 for determining the shape during operation of the polygon mirror 1; and a pulse generator 11 for adjusting a timing between the operation of the polygon mirror 1 and emission from the semiconductor laser 2. In the device, it is determined by the computer 10 whether the peak wavenumber is below the threshold, and thereby only the interference fringe having the peak wavenumber below the threshold is used for the measurement.