An atomospheric exposure test has been conducted since Sept. 26, 1994 at a five member family steel framed prefabricated house in Nogi-chou, Shimotuga-gun, Tochigi prefecture. The ACM (Atmospheric Corrosion Monitor) corrosion sensors and zinc-coated steel coupons are exposed in nine indoor sites-ceilings, floors and wall cavities-and one outdoor site sheltered from rain. Three year of measured results are reported here. The ACM sensor outputs galvanic current, I, between carbon steel and silver, integral of which over a day -daily average electricity, Q- is related to corrosion rates of carbon steel and zinc to be used as a measure of corrosivity for each site. The sensor output, I, is found to depend not only on relative humidity, RH, but also on amount of deposits on the sensor, which, expressed as equivalent amount of deposited sea salt, Ws, is determined by referencing to I vs. RH relationship observed under various predetermined amounts of deposited sea salt in laboratory test. Corrosion rates of carbon steel and zinc at their indoor sites are around one fifth of the outdoor one. Such lower corrosion rates at the indoor sites are attributed mainly to lower Ws values which are around one tenth of the outdoor one. Moreover, amount of sulfur trioxide collected for a year is 0.001mg SO3/100cm2/day at an indoors site, one twenty sixth of the outdoor one.