In order to clarify a dominant factor occupying oxide-ion conductivity, electrical conductivity was measured for the fluorite-type solid-solution systems, (Y1-xMx)(3)NbO7 (M = Ca, Mg) and Y-3(Nb1-xMx')O-7 (M' = Zr and Ce) by means of the ac two-probe method, A solubility limit of dopant was around x = 0.1 for each system. Activation energy for the electrical conduction was estimated to be 117 kJ/mol for Y3NbO7 and decreased to the range 94-404 kJ/mol for the solid-solution samples. The compositional change in the conductivity could be interpreted neither by oxygen vacancy concentration nor by substituted ion radius. Resultantly, it was found that the oxide-ion conduction was strongly dependent on the unit cell free volume in the present study.