A stop pattern setting part 203 is provided which permits setting therein a plurality of patterns for stopping the test pattern updating operation of a test pattern generator 210, and upon each generation of the test patters set in the stop pattern setting part 203, the test pattern generator 210 is stopped from the pattern updating operation. Each time the test pattern stops, a stop signal is applied to an electron beam probe system 300, causing it to start an image data acquiring operation. Upon completion of the image data acquisition, a write completion signal generating part 308 generates a write completion signal, which is applied to the test pattern generator 210 to cause it to resume the pattern updating operation. By applying different test patterns to a device under test alternately with each other and displaying image data of the difference between resulting pieces of image data, a potential contrast image can be improved.