The oxygen flux density of Ce0.8Gd0.2O1.9–x vol% MnFe2O4 (CGO-xMFO) composite-type ceramics membranes has been investigated. The samples and reforming catalysts were prepared by the Pechini process. For the CGO-xMFO composites, oxygen permeation was observed even at x = 3 vol%, presumably due to the presence of grain boundary phases. For CGO-15MFO, the n-type electronic conductivity was found to be dominant at 900°C or higher. The thickness dependence of jO2 revealed that surface exchange kinetics was significantly involved in the case of the membrane thickness of L < 0.5 mm. The highest oxygen flux density of 10 μmol·cm−2·s−1 was achieved for CGO-15MFO with the 10 mass% Ni-Pr:CeO2 catalyst (L = 0.25 mm) at 1000°C and a flow rate of 270 sccm.