Rapid estimation of the epicentral distance and magnitude is of fundamental importance for early earthquake detection and warning systems. We present a novel method of estimating the epicentral distance from a single seismic record in a short amount of time. In order to quantitatively evaluate the difference in observed seismic waveforms, we introduced a simple function with the form of Bt·exp(-At) and determined A and B in terms of the least-squares method by fitting this function to the initial part of the waveform envelope. We have found that log B is inversely proportional to log Δ, where Δ is the epicentral distance. This relation holds true regardless of the earthquake magnitude. By using this relation, we can roughly estimate the epicentral distance nearly immediately after the P-wave arrival. Then, we can readily estimate the magnitude from the maximum amplitude observed within a given short time interval after the P-wave arrival by using an empirical magnitude-amplitude relation that includes the epicentral distance as a parameter.