Photoluminescence (PL) due to Ce3+ in SiO2 glass is strongly influenced by co-doping of phosphorus (P), whose features such as peak energy and fluorescence lifetime are very close to those in phosphate glass. The origin for this striking effect of the P co-doping has been clarified through the elucidation of the coordination structure around a Ce3+ by pulsed EPR ESEEM spectroscopy. Analysis of the observed ESEEM pattern with simulation revealed that the number of P-ions in the coordination sphere is larger by an order of magnitude than that expected from the uniform distribution of P, demonstrating the formation of a solvation shell structure. It has been found that two types of the coordination manner of P-O bonds co-exist in the solvation structure, corner-sharing and edge-sharing. This solvation shell structure is almost the same as that for the calcium phosphate glass.