First, the phenomena relating to the effectiveness of the spoiler and a law underlying the phenomena were examined by experiments. To be more specific, quantitative study was made on the phenomenon that the spoiler wake was affected by the portion of airfoil behind the spoiler, and on the tendency for the spoiler wake region to shrink. The former phenomenon was designated as "wake plate effect", and the latter, "reattach tendency". Next, a wake model reasonably representing the law relating to the phenomena, and an airfoil model with a normal spoiler which was obtained from a circle through the conformal transformation were combined into one mathematical model. Using this mathematical model, a theory on spoilers was developed on the basis of the two dimensional potential theory and was simplified to an approximate theory for practical purposes, which was proved to agree fairly closely with experiments.