  In this article, we derive the formula for the Casimir force acting on a
piston made of real material moving inside a perfectly conducting rectangular
box. It is shown that by taking suitable limits, one recovers the formula for
the Casimir force acting on a perfectly conducting piston or an infinitely
permeable piston. Lifshitz formula for finite temperature Casimir force acting
on parallel plates made of real materials is re-derived by considering the
five-layer model in the context of piston approach. It is observed that the
divergences of the Casimir force will only cancel under certain conditions, for
example, when the regions separated by the plates are filled with media of the
same refractive index.
