Kagom\'e in triangular lattice: electronic state of CoO_2 layer with hexagonal structure

The electronic state in layered cobalt oxides with hexagonal structure is examined. We find that the electronic structure reflects the nature of the Kagom\'e lattice hidden in the CoO_2 layer which consists of stacked triangular lattices of oxygen ions and of cobalt ions. A fundamental model for the electron system is proposed and the mechanism of the unique transport and magnetic properties of the cobalt oxides are discussed in the light of the model.