Proton dynamics in hydrogen-bonded systems, in particular, dissociation and recombination of water molecules are basic processes in many chemical and life systems. In this article, we discuss the pressure dependence of electric conductivity of high-pressure ices VII and X, which may exist in astronomical icy bodies, from the following three viewpoints: (1) molecular dynamics simulation of the hydrogen and oxygen atoms, (2) thermodynamic transport theory of ionic and rotational defects of ice, (3) lattice model of proton hopping.