We present a new method of estimating the epicentral distance based on the slope of an initial part of P wave arrival. In order to express the difference in waveforms quantitatively, we introduced a simple function with the form of Bt exp(-At) and determined unknown parameters A and B by the least squares method to fit the function to observed seismic wave data. As a result, we found that log B is closely related to epicentral distance D. This relation seems to be true regardless of the earthquake magnitude. By using this relation, we can approximately estimate the epicentral distance immediately after the arrival of P wave. By using log B in place of D, we can estimate the magnitude easily from a formula similar to the conventional formula M=a log V_max + b log D + c.

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