  We derive the discretized Maxwell's equations using the discrete variational
derivative method (DVDM), calculate the evolution equation of the constraint,
and confirm that the equation is satisfied at the discrete level. Numerical
simulations showed that the results obtained by the DVDM are superior to those
obtained by the Crank-Nicolson scheme. In addition, we study the two types of
the discretized Maxwell's equations by the DVDM and conclude that if the
evolution equation of the constraint is not conserved at the discrete level,
then the numerical results are also unstable.
