The Impact Vibration Test is now a recognized method for evaluating the soundness of bridge substructures. This method focuses on the natural frequency of the pier under test, since it clearly shows the condition of the bearing capacity of the foundation and the soundness of the pier. The measured and standard values of the pier's natural frequency are compared for evaluation, and it is judged to be "sound" when the former is sufficiently higher than the latter, and as unsound otherwise. The standard value of the natural frequency is calculated by statistical analysis using several parameters, such as the pier height, pier width and the weight of the superstructure, although these parameters are highly correlated with the natural frequency. But the conventional standard value calculation formula does not have physical significance at all. Entirely based on a design concept for foundations, this paper introduces a new and progressive method for the standard value of the natural frequency of wooden pile foundations.