Makuhari-Messe Convention Center building is located in a seashore area, 500m from the sea, at Chiba city in Chiba prefecture. Its roof and eaves are made of 22Cr-0.8Mo stainless steel. To evaluate the corrosivity of atmosphere at the eaves in this building, ACM(Atmospheric Corrosion Monitor) sensors and 22Cr-0.8Mo steel specimens were exposed at five sites, where Sites 1 and 2 were located at the south-west side of the building, Side A, facing the sea, and Sites 3, 4 and 5 were located at the north-east side, Side B, behind the sea. They had been exposed since '94.11 and renewed every 1-4 months until '96.11. The outputs of ACM sensors were measured every 10 minutes together with temperature and relative humidity (RH). The amount of deposited sea salt, Ws, was also determined based on ACM sensor outputs combined with RH data. The 22Cr-0.8Mo steel specimens were rusted only at Sites 1 and 2 when Ws increased more than 1g/m2, during a single exposure period, '95.4.18-'95.6.22, in the whole exposure period of 2 years ('94.11-'96.11). The day, when Ws exceeded 1g/m2 and make the steel rusted, could be identified based on the effective coefficient of wind-force energy in a day, α*D.