  Recently, a new energy density formula for electromagnetic waves in the
metamaterial consisting of arrays of conducting wires and split-ring-resonators
(SRR) has been derived [Phys. Rev. E 80, 046601 (2009), arXiv:0909.1043].
According to that formula, the positiveness is obvious for the electric part of
the energy density but not clear for the magnetic part. In this paper, I show
that the magnetic energy density is also positively definite.
