  We study Casimir effect in equilibrium and non-equilibrium photon gas in the
frame of quantum kinetic theory for $U(1)$ gauge field. We derive first the
transport, constraint and gauge fixing equations for the photon number
distribution from Maxwell's equations, and then calculate the energy variation
and Casimir force for a finite system by considering boundary condition on the
surface of the system. The Casimir force in vacuum is suppressed by the thermal
motion of photons in equilibrium state, when considering two adiabatic plates.
In non-equilibrium state, the photon induced Casimir force oscillates and
decays with time and finally disappears.
