  We present a study of the Casimir effect in an imperfect (mean-field) Bose
gas contained between two infinite parallel plane walls. The derivation of the
Casimir force follows from the calculation of the excess grand canonical free
energy density under periodic, Dirichlet, and Neumann boundary conditions with
the use of the steepest descent method. In the one-phase region the force
decays exponentially fast when distance $D$ between the walls tends to
infinity. When Bose-Einstein condensation point is approached the decay length
in the exponential law diverges with critical exponent $\nu_{IMP}=1$, which
differs from the perfect gas case where $\nu_{P}=1/2$. In the two-phase region
the Casimir force is long-range, and decays following the power law $D^{-3}$,
with the same amplitude as in the perfect gas.
