This study was carried out to investigate effects of iron content on the quality of soy sauce, bean paste and red pepper paste, and to elucidate the origin of iron and change of the contents during production processes. For the first step, the iron contents in commercial soy sauce and changes of the contents during brewing process were determined. The results obtained were as follows. 1, Iron contents of raw materials were 108 ppm in soy bean, 133ppm in defatted soy bean, 79 ppm in wheat, 5 ppm in sodium chloride, 58 ppm in seed koji, 300-2000 ppm in spore of Aspergillus oryzae, 240 ppm in wheat gluten, 20 ppm in sodium carbonate (above figures were of dry weight basis), 6 ppm in hydrochloric acid, 18 ppm in caramel and 0.3ppm in brewing water respectively. 2, Iron contents in koji were 200-240 ppm (as dry weight basis) and increased, more or less, in progress of koji-making period. 3. Iron contents in the mashes during fermentation were 40 rpm after 1 month, 43-47 ppm after 3 months and 49-62ppm after 6 months. 4. In chemical soy sauce, the iron content was 159 ppm after hydrolysis of wheat gluten with hydrochloric acid, and 184 ppm after neutralization. 5. Higher iron contents were detected both in fermented and chemical soy sauce when the concentration of total nitrogen increased, but the levels were higher in chemical soy sauce than in fermented one at the same concentration of total nitrogen. 6. In the case of fermented soy sauce, the iron content in the filtrate was decreased by press-filtration, but no significant change was found between before and after heat-sterilization. 7. Iron contents in commercial soy sauce were varied with the producers, however, the average value was 62.7 ppm as calculated as 1.0 percent of total nitrogen. And the average level of iron in home-made soy sauce produced by conventional method was 37.68 ppm.