Relation between corrosion resistance of steel and NaCl concentration has been studied by wet and dry corrosion test. Corrosion products of steel were directly analyzed in wet and dry corrosion test using In-situ X-ray diffraction equipment. Corrosion rate of steel increased with increasing NaCl concentration. The reduction reaction of rust accelerated corrosion of steel. Ratio of β-FeOOH in rust tends to increase with increasing NaCl concentration. β-FeOOH is reduced to amorphous intermediate oxide in wet cycle, and reproduced in dry one. Amorphous intermediate oxide is reduced to Fe₃O₄ in constant current reduction, but hardly reduced to that in actual wet and dry corrosion cycle.