Highly porous gallium oxide was synthesized by reconstructing its surface and body with mesopores and macropores. For the first time, the efficient photocatalytic conversion of CO2 into a high energy carrier, CH4, using the porous gallium oxide was realized without any co-particle or sacrificial reagent. The enhanced photocatalytic activity is mainly attributed to the 300% higher CO2 adsorption capacity, as well as the 200% increased surface area, compared to the bulk nanoparticles. Furthermore, we propose the new reaction pathway based on the result that the carbon dioxide was converted directly into methane without going through carbon monoxide intermediates.