  We examine the Casimir energy of 5D electro-magnetism in the recent
standpoint. Z$_2$ symmetry is taken into account. After confirming the
consistency with the past result, we do new things based on a new
regularization. The regularization is based on the minimal area principle and
the regularized configuration is the {\it sphere lattice}. We do it not in the
Kaluza-Klein expanded form but in the closed form. The formalism is based on
the heat-kernel approach using the position/momentum propagator. A useful
expression of the Casimir energy, in terms of the P/M propagator, is obtained.
Renormalization flow is realized as the change along the extra-axis.
