NOVELTY - The photo-catalytic device comprises a main body converting carbon dioxide reaction gas delivered by a light source into generating gas of methane by chemical action using an optical catalyst. A gas reaction unit is connected to the main body and injects the carbon dioxide reaction gas. A gas generating unit is connected to the main body and discharges the generated methane generating gas. A vacuum pump unit is connected to the main body and gives a vacuum. A multi-photon absorption window is equipped in the main body, where window includes a convex lens that is integrated in light source.    USE - The photo-catalytic device is useful for carbon dioxide methanation reaction (claimed).    ADVANTAGE - The photo-catalytic device has improved energy conversion efficiency, and can facilitated and analyze the reaction quantities with correct result using analytical equipment.    DETAILED DESCRIPTION - The photo-catalytic device comprises a main body for converting carbon dioxide reaction gas delivered by a light source into generating gas of methane by chemical action using an optical catalyst. A gas reaction unit is connected to the main body and injects the carbon dioxide reaction gas. A gas generating unit is connected to the main body and discharges the generated methane generating gas. A vacuum pump unit is connected to the main body and gives a vacuum. A multi-photon absorption window is equipped in the main body, where the window includes a convex lens integrated in the light source. The multi-photon absorption window is installed at top and bottom surfaces of the main body. The main body has a flat transparent window integrated with top and bottom surfaces. The convex lens is separated from and connected to the top surface of flat transparent window. The gas reaction unit, the gas generating part, the vacuum pump unit and the separation unit are connected with the main body through a fluid connection. An opening and closing valve is located in a pipe. The main body part has a separate unit connected to the reaction gas unit, the generating gas unit, the vacuum pump unit and separation unit through the fluid connection.