1. A sample dimension measuring method for scanning an electron beam on a sample and measuring dimensions of patterns formed on said sample based on detection of electrons emitted from scanned sites, the method comprising steps of: measuring the patterns at a plurality of different sites a plurality of times to obtain a plurality of measurements; calculating a mean value of variance calculated based on a difference between measured value data in the plurality of measurements and a curve representative of a reduction of the measured value data with an increase of the number of measurements, so that the curve is formed to make the mean value of variance small; storing the formed curve with respect to each type of pattern; and measuring a dimension of each of pattern before reduction in dimension, based on the stored curve.