The solubility limit of Ce3+ ions in Ce-substituted fibrous yttrium iron garnet (Y3-xCexFeO12; Ce:YIG) single crystals grown by a floating zone (FZ) method in an oxygen atmosphere was approximately x = 0.3, and the Faraday rotation was only about -810 degrees /cm at lambda = 1.55 mum. We investigated the effect of growth atmosphere on the solubility limit of Ce3+ ions in order to increase the content of Ce3+ ions in Ce:YIG crystal. The solubility limit of Ce3+ ions in the crystal was increased to approximately x=0.5 by growth in a nitrogen atmosphere, and Faraday rotation was also linearly increased (about -1310 degrees /cm at lambda =1.551 mum), This was thought to be due to the Ce4+ ions (source CeO2) oxidizing Fe2+ ions to Fe3+ ions and Ce4+ ions being reduced to Ce3+ ones. We expect Ce:YIG crystals to be used as new Faraday rotator materials applicable for optical isolators and circulators.