We analyzed microstructure to understand the initial scaling process on the stainless steels immersed at hot spring water in Obama town, Unzen city, Nagasaki pref., Japan. The adhesion amounts of scale kept increasing during the immersion testing. Columnar calcium carbonate and particulate magnesium silicate adhered on the stainless steel in both the aeration tank and in the drain located posterior to it. They kept growing during the immersion testing, but the growth rate was slow. Their scales were aggregated by the effect of magnesium silicate such as adhesive, and formed to large aggregation scales. Net-like magnesium silicate adhered only in the aeration tank. It formed on the overall stainless steel surface immersed for 3 days. The growth rate of magnesium silicate was faster than that of particulate scales, and was firstly covered with the surface of stainless steel. Though NaCl is unsaturated in the hot spring water, elements of Na and CI were detected in net-like magnesium silicate in the EDS analysis. NaCl may promote the precipitation of magnesium silicate.