  A $\theta$ term in lattice field theory causes the sign problem in Monte
Carlo simulations. This problem can be circumvented by Fourier-transforming the
topological charge distribution $P(Q)$. This strategy, however, has a
limitation, because errors of $P(Q)$ prevent one from calculating the partition
function
  ${\cal Z}(\theta)$ properly for large volumes. This is called flattening. As
an alternative approach to the Fourier method, we utilize the maximum entropy
method (MEM) to calculate ${\cal Z}(\theta)$. We apply the MEM to Monte Carlo
data of the CP$^3$ model. It is found that in the non-flattening case, the
result of the MEM agrees with that of the Fourier transform, while in the
flattening case, the MEM gives smooth
  ${\cal Z}(\theta)$.
