A visual masking experiment was carried out to analyze the processing stages in the human visual system. The experiment was divided into four conditions depending on the kinds of test and mask stimuli involved. When a test stimulus was a Kanji character, a mask stimulus was either another character or a flash. When a test stimulus was an English alphabet letter, a mask stimulus was either another letter or a flash. Comparing results for four conditions, the stages involved in the flash mask experiment were determined. It was also shown that the first character processing stage in the human visual system was the contour formation process. It produces the contour pattern from the retinal image by filtering the brightness component or by extracting a brightness difference component. The estimated processing time for contour formation was about 20 msec for a letter and about 30 msec for a Kanji character.