With use of diol, alkoxysilane, and a catalytic amount of trimethylsilyl trifluoromethanesulfonate (TMSOTf), carbonyl compounds are converted to acetals in good yields under mild conditions. This procedure, which was carried out without synthesizing the silylated diols, is a more convenient adaptation of Noyori's method. This acetalization applies to not only simple but also conjugated carbonyl compounds. Moreover, various TMS compounds, including solid supported compounds, are effective for this method instead of alkoxylsilane.