  We have studied the statistical mechanics of money circulation in a closed
economic system. An explicit statistical formulation of the circulation
velocity of money is presented for the first time by introducing the concept of
holding time of money. The result indicates that the velocity is governed by
behavior patterns of economic agents. Computer simulations have been carried
out in order to demonstrate the shape of the holding time distribution. We find
that, money circulation is a Poisson process in which the holding time
probability distribution follows a type of Gamma distribution, and the velocity
of money depends on the share for exchange and the number of agents.
