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We have developed the apparatus for monitoring changes in humidity in the test environment and mass of the rusted steel in order to detect the moisture absorbed into and desorbed out of the steel rust. The rusted steel was set in the container in which the air was controlled to be dry or humid. Change in mass of the rusted steel due to absorption or desorption of moisture was continuously monitored by an electric analytical microbalance, and humidity in the container was spontaneously determined by a digital hygrometer. In the air of a relative humidity (RH) more than 90 , mass of the rusted steel increased rapidly and parabolically due to absorption of moisture into the rust, and then increased gradually and linearly due to adsorption of moisture on the surface. As the test lasted, the mass oscillated ; sudden decrease in mass corresponded to falling a water droplet with some pieces of the rust, and linear increase in mass thereafter did to re-adsorbing moisture. In the case of RH less than 90 , mass change was similar to that of RH more than 90 , but there was no falling of a water droplet from the rusted steel. It is revealed from the long-term test in the air of RH 88 that mass of moisture absorbed into the rust was independent of test time and that mass of new rust increased linearly with test time. In addition, it is suggested that the mass increase rate, that is corrosion rate, corresponds to the cathodic reaction rate limited by diffusion of dissolved oxygen in the thick water film.Keywords : rusted steel, dry and humid air, absorption of moisture, desorption of moisture, mass change, relative humidity, dissolved oxygen 564 8680 3 3 35 (3 3 35 Yamate-cho, Suita,