A new pretreatment of catalyst, deposition-removal (D-R) treatment, is proposed to suppress carbonaceous depositions on Ni/Al2O3 catalyst for carbon dioxide reforming of methane. This treatment is based on the hypothesis that active cores forming carbon whiskers are different from surface active sites for the main reaction. In the deposition step, active cores of nickel are detached from the bulk nickel surface as growing cores in carbon whiskers. In the removal step, the carbon whiskers are removed from the nickel surface. These two steps are repeated by flowing pure methane gas and then pure carbon dioxide gas. The thermogravimetric (TG) measurements showed that the repeated D-R treatments reduced the carbonaceous deposition. Carbon monoxide adsorption measurements showed that the nickel surface area was decreased by the treatments. The reaction rate and the turnover frequency were increased by the treatments. The increase in the activity was explained by the development of active sites on newly exposed nickel layers which strongly interacted with the support.