PURPOSE: To obtain an image-processing device which can measure a grain size number accurately regardless of adhesion of dust, dirt, etc., on a surface to be inspected of a sample and a break of a crystalline grain boundary.

CONSTITUTION: In an image-processing device which measures a crystalline grain size number according to an enlarged image of a microscope, a projection data with a predetermined constant width in vertical and horizontal directions of a binary image data on a surface to be inspected is created by a projection data creation circuit 15. A significance of a picture element color is determined by a projection data judging circuit 17 according to the number of distribution of white or black picture elements in the direction of each line from the projection data and at the same time a binary- coded projection data is created. A point which changes from 0 to 1 in the direction of each line of the binary-coded projection data is detected by a differentiation circuit 9 and the number of change points is counted by a counter 10. An average calculation target data selection circuit 23 excludes i creation regions below a maximum value and j creation regions above a minimum value for each direction out of the number of crystalline particles for each creation region in horizontal and vertical directions which is counted by the counter 10, and then averages the number of crystalline particles of the remaining creation region and outputs it to a grain size number calculation circuit 11.