Oxide growth on Si wafers during a high-temperature N-2 anneal in cantilever-type horizontal furnaces was investigated, with special attention given to the influence of air in a clean room on the distribution of wafer oxide thickness. Two distinct regions can be found in the thickness distribution characteristics which correspond to the wafer positions on a quartz boat : the varying thickness region near the quartz tube gas outlet (A region), and the constant thickness region within the entire quartz tube (B region). To identify the differences in the thickness distribution between the two regions, the change in thickness was examined for various factors, such as carrier gas flow rate. As a result, oxide growth in the A region was found to occur when the carrier gas flow rate is small during the quartz cap closing and was caused by the air remaining near the quartz tube gas outlet because of the small carrier gas flow rate. On the other hand, oxide growth in the B region was found to occur during the quartz boat withdrawal and was caused by air entering the quartz tube from the open gas outlet and a higher wafer temperature compared to the quartz boat insertion. Consequently, it was concluded that the most effective method to suppress oxide growth in the two regions is by increasing the carrier gas flow rate and by cooling in an airtight place.