A deoxidizer includes a porous body of fluorite-type cerium oxide represented by CeOx (where x is a positive number smaller than 2) and having a reversible oxygen deficiency. The deoxidizer has a specific surface area of 0.6 to 1.8 m2/g and a pore median diameter of 1.6 to 5.3 μm. The cerium oxide used for the deoxidizer is produced by: firing a cerium-containing salt in the atmosphere at 500° C. to 1400° C. for 1 to 20 hours, to produce fluorite-type cerium oxide composed of a porous body; and firing the cerium oxide at 700° C. to 1100° C. for 1 to 3 hours in a reducing atmosphere having a hydrogen concentration equal to or above the lower explosive limit, to remove oxygen from the cerium oxide and produce the fluorite-type cerium oxide represented by CeOx and having a reversible oxygen deficiency.