In a scanning electron microscope, if a failure is caused to occur in a SEM image by the influence of a disturbance such as magnetic field or vibration inside and from outside the device, the cause is identified simply and accurately using this SEM image. There is provided a measurement technique whose measurement accuracy is not influenced by a roughness of SEM image pattern. A one-dimensional scanning is performed in a scanning-line direction (X direction) by setting the Y-direction scanning gain at zero at the time of acquiring the SEM image, and a two-dimensional image is created by arranging image information, which is obtained by the scanning, in a time-series manner in the Y direction. A shift-amount data on the two-dimensional image is acquired using a correlation function, and the magnetic field or vibration included within the SEM image is measured by a frequency analysis of the data.