A scanning electron microscope and an optical-condition setting method are provided. The optical condition allows the suppression of a lowering in the measurement and inspection accuracy caused by the influence of electrification, even if there are a large number of measurement and inspection points. A pattern on a sample is measured based on the detection of electrons by scanning the sample surface with an electron beam. A change in measurement values relative to the number of measurements is determined from the measurement values at a plurality of measurement points on the sample, and the sample-surface electric field is controlled so that the inclination of the change becomes equal to zero, or becomes close to zero.