Surface area and methane conversion activity in a complete combustion of methane of LaCoO3 and LaMnO3 perovskite-type oxides are increased very markedly by treating the perovskite-type oxides with water under autogeneous pressure (at 125-300 degrees C) and/or with steam (at 350-800 degrees C) at atmospheric pressure for 4h. The methane combustion activity per unit surface is, however, decreased markedly due to the hydrothermal water treatment of LaCoO3, but it is influenced only to a small extent because of the steam treatment of LaCoO3 and the hydrothermal water treatment of LaMnO3.