The selective oxidation activity of the molybdena thin layer loaded on SnO2 which was calcined at 773-1173 K was studied. MoO3 spread readily on SnO2, and covered the surface of support with ca. 80% coverage at most independent of the calcination temperature of tin oxide. On the other hand, the rum-over frequency depended on the calcination temperature, i.e., the high TOF was kept on the catalyst with the SnO2 calcined below 973 K, but it decreased with the SnO2 calcined above 1073 K, Characterization of the catalysts revealed relations between the catalytic activity and the property. i,e.. reducibility and solid acidity. In particular, the strong dependence upon the solid acidity was observed. The acid sites were generated on the molybdenum oxide loaded on tin oxide calcined below 973 K, and possibly identified as the Bronsted acid sites. Methanol was adsorbed more weakly on the acid sites than on exposed surface of SnO2, and converted into formaldehyde selectively.