A method for processing images using an observation device including: an irradiation device (2, 3, 4) for irradiating an object to be observed with electron beams or X-rays; a detector for detecting responses of the object occurring as a result of irradiation thereof; a holder unit for holding the object; a tilting device capable of arbitrarily setting a tilt angle of the object, comprising the following steps of: tilting the object at predetermined angle steps; storing the images which are obtained as 1st to Nth image data observed at respective tilt angles; performing alignment calculation to align the stored 1st to Nth image data; generating an initial reconstructed image from a projection image of the object, characterized by: projecting the reconstructed image at predetermined angles to generate and store 1st' to Nth' image data; performing error calculation to calculate errors in corresponding pixels between the 1st to Nth images and the 1st' to Nth' images with a set number N of iteration steps; determining a processing priority order on the basis of the integrated value of all residual errors associated with each quantum unit in descending order of the integrated values for all quantum units; performing density calculation with a set number M of iteration steps to calculate a density of pixels for each gradation level by providing a section around a quantum unit to be given a certain degree of priority and determining the ratio between the number of pixels within the section and the number of quantum units of the same gradation level as the quantum unit within the section and the quantum units are prioritized for movement in the order of lowest to highest in terms of the above ratio, so as to determine a processing priority order on the basis of the densities; and changing a gray level of each pixel in the reconstructed image in each of the processing priority orders. tilting the object at predetermined angle steps; storing the images which are obtained as 1st to Nth image data observed at respective tilt angles; performing alignment calculation to align the stored 1st to Nth image data; generating an initial reconstructed image from a projection image of the object, characterized by: projecting the reconstructed image at predetermined angles to generate and store 1st' to Nth' image data; performing error calculation to calculate errors in corresponding pixels between the 1st to Nth images and the 1st' to Nth' images with a set number N of iteration steps; determining a processing priority order on the basis of the integrated value of all residual errors associated with each quantum unit in descending order of the integrated values for all quantum units; performing density calculation with a set number M of iteration steps to calculate a density of pixels for each gradation level by providing a section around a quantum unit to be given a certain degree of priority and determining the ratio between the number of pixels within the section and the number of quantum units of the same gradation level as the quantum unit within the section and the quantum units are prioritized for movement in the order of lowest to highest in terms of the above ratio, so as to determine a processing priority order on the basis of the densities; and changing a gray level of each pixel in the reconstructed image in each of the processing priority orders. projecting the reconstructed image at predetermined angles to generate and store 1st' to Nth' image data; performing error calculation to calculate errors in corresponding pixels between the 1st to Nth images and the 1st' to Nth' images with a set number N of iteration steps; determining a processing priority order on the basis of the integrated value of all residual errors associated with each quantum unit in descending order of the integrated values for all quantum units; performing density calculation with a set number M of iteration steps to calculate a density of pixels for each gradation level by providing a section around a quantum unit to be given a certain degree of priority and determining the ratio between the number of pixels within the section and the number of quantum units of the same gradation level as the quantum unit within the section and the quantum units are prioritized for movement in the order of lowest to highest in terms of the above ratio, so as to determine a processing priority order on the basis of the densities; and changing a gray level of each pixel in the reconstructed image in each of the processing priority orders.