1. A scanning electron microscope in which a specimen is irradiated with a primary electron beam to acquire an image of the specimen, the scanning electron microscope comprising: an energy filter for catching electrons having at least a predetermined energy level among the electrons emitted from the specimen; time constant extraction means for extracting the time constant of a temporal change curve indicative of the relationship of the signal magnitude of the electrons caught by the energy filter with the electron beam irradiation time; and scanning order determination means for determining the scanning order in which the electron beam scans the specimen based on the extracted time constant, wherein the specimen is scanned and observed in the determined scanning order.