According to one embodiment, a diffraction signal of each of a plurality of reference structures provided on a substrate is acquired. The diffraction signals are classified based on a similarity to generate a first data map. The diffraction signals in the first data map are used to interpolate data between the diffraction signals to generate a first interpolation data map. An actual dimension of each of the plurality of reference structures is measured. The actual dimensions are arranged to correspond to the diffraction signals of the first data map to generate a second data map. The actual dimensions in the second data map are used to interpolate data between the actual dimensions to generate a second interpolation data map. The first interpolation data map and the second interpolation data map are used to derive a calculation formula by which the actual dimension is obtained from the diffraction signal.