  In layered single band systems, the interlayer conductivity depends on the
orientation of the in-plane magnetic field and takes maximum values when the
magnetic field is perpendicular to flat regions of the Fermi surface. Extending
this known results to multi-band systems, we propose an experiment to extract
information about their Fermi surface topology. We discuss application of the
formula to a FeAs-based superconductor, LaFeAsO$_{1-x}$F$_x$. We show that the
magnetically ordered state in the parent compound is clearly distinguished from
the paramagnetic state by the oscillation period in the interlayer
conductivity. We demonstrate that evolution of the Fermi surface topology by
changing the doping concentration is reflected to the interlayer conductivity
oscillation patterns.
