Ethylene can be photooxidized selectively with water as an oxidant via irradiation with UV light over copper oxide supported on silica after evacuation at 973 K. Ethylene oxide was formed by irradiation of UV-filtered light at a wavelength longer than 280 nm, and a high selectivity to ethylene oxide (82.5%) with an ethylene conversion of 1.6% was obtained at 278 K for 5 h of irradiation. However, irradiation of UV-unfiltered light results in the disappearance of ethylene oxide, while carbon dioxide, acetaldehyde, ethanol, ethane, and C-3 and C-4 hydrocarbons are formed. X-ray absorption near-edge structure (XANES) spectroscopy showed the presence of Cu+ cations in the pore structure of the catalyst, suggesting that a mild oxidant can be generated from water on Cu+ sites by UV light irradiation.