  We consider an infinite-range Ising model under the Glauber dynamics and
determine the finite-size effect on the distribution of two spin variables as a
perturbation of $O \left( 1/N \right)$. Based on several considerations,
ordinary differential equations are derived for describing the time development
of both a two-body correlation and the autocorrelation function of
magnetization. The results of the calculation fit the simulation results,
unless the perturbation theory breaks down because of critical phenomena or
magnetization reversal.
