  Analyzing historical data of price indices we find an extraordinary growth
phenomenon in several examples of hyper-inflation in which price changes are
approximated nicely by double-exponential functions of time. In order to
explain such behavior we introduce the general coarse-graining technique in
physics, the Monte Carlo renormalization group method, to the price dynamics.
Starting from a microscopic stochastic equation describing dealers' actions in
open markets we obtain a macroscopic noiseless equation of price consistent
with the observation. The effect of auto-catalytic shortening of characteristic
time caused by mob psychology is shown to be responsible for the
double-exponential behavior.
