To clarify the ionic conduction of Ba(Pr0.6Gd0.4)O3-alpha, the electrical conductivity was measured in moist air and hydrogen atmospheres. The ionic transport number was estimated by a steam concentration cell, a hydrogen permeation cell, and a fuel cell. Temperature dependence of the conductivity in a moist air atmosphere differed from that in a moist H-2 atmosphere. The conductivity under reducing conditions increases with time according to the crystal structure change of Ba(Pr0.6Gd0.4)O3-alpha due to oxygen loss in the lattice site. Under moist air conditions, the dominant conduction species in the Ba(Pr0.6Gd0.4)O3-alpha electrolyte were mainly holes. However, the species changed into proton, oxide ion and hole by means of structure change in a reducing atmosphere. It was considered that the ionic conduction occurred due to the crystal structure change. It was predicted that this peculiarity of the conductivity of barium praseodymium oxide influenced the nonstoichiometric behaviour of Pr in the crystal structure.