  The Huttner$-$Barnett model is extended to a magnetodielectric medium by
adding a new matter field to this model. The eigenoperators for the coupled
system are calculated and electromagnetic field is written in terms of these
operators. The electric and magnetic susceptibility of the medium are
explicitly derived and shown to satisfy the Kramers$-$Kronig relations. It is
shown that the results obtained in this model are equivalent to the results
obtained from the phenomenological methods.
