Dielectric relaxations were investigated for the solid-solution system Ce_1-xSm_xO_2-δ(0.0≦x≦0.5), which is a typical oxide-ion conductor. The dielectric constants showed anomalously large values at low frequencies and high temperatures. Computer simulation clarified that the anomalously large dielectric constant (ε_r′) originated from the superimposition of both Debye-type polarization and interfacial polarization between electrolyte and electrode. Two kinds of Debye-type relaxation observed were ascribed to defect associates, (Sm_Ce′-V_O^••)^• and (Sm_Ce′-V_O^••-Sm_Ce′)^×. The Debye-type polarization was also confirmed by analyzing the dielectric loss factor (ε_r″). When the oxide-ion conductivity decreased in the heavy Sm-doped samples, the Debye-type polarization also disappeared, suggesting that ordering of oxygen vacancy suppressed the electric field response of Debye-type polarization.