An object of the present invention is to provide, for the production of isobutene, a high-yielding, highly-selective, and long-term stable production process of isobutene from TBA. With respect to the production of TBA, an object of the present invention is to provide a TBA production process in which, through long-term stable maintenance of a high reaction activity, long-term continuous operation is enabled and the productivity is improved. The present invention discloses a process for producing isobutene that employs a dehydration temperature of from 200 to 450° C. in use of an alumina catalyst that contains a Na content of 0.6% by weight or less in terms of NaO2 and a Na content of 0.4% by weight in terms of NaO2, and has a specific surface area of from 200 to 600 m2/g.