The inelastic neutron scattering measurements of superionic glass system (AgI)x(Ag2S)x(AgPO3)1-2x reveal that an excess intensity around 3 meV, the so-called boson peak, increases with the dopant salt concentration x. The composition dependence of boson peak intensity has a systematic relation with ionic dc conductivity. These correlated features are discussed from the viewpoint of free spaces involved with the expansion of network structure by salt doping. Cooperative dynamical properties concerned with free spaces contribute to the enhancement of hopping probability of Ag+ ions. Successive hopping is promoted in order to keep electric neutrality, where the hopping events of charged ion microscopically violates the electric neutrality. These processes should result in the high ionic conductivity of superionic glasses.