Photoluminescence of cerium (Ce) in silica was investigated by putting Ce ion into pores of mesoporous silica by soaking the silica in solutions of trivalent or tetravalent Ce ion followed by drying. It has been shown that no luminescence was observed for -soaked samples. But for -soaked samples without any reduction treatment, very strong luminescence was unexpectedly observed. There are two luminescence peaks at around 345 and 665 nm. The intensities at the peaks increase with rise of ion concentration in soaking solution up to 0.15 M beyond which higher concentration leads to decrease of the intensity. These facts were explained in terms of ion exchange between ion and silicon in the silica network on pore walls and valence variation of Ce in the silica network during drying.