In Direct Internal Reforming Molten Carbonate Fuel Cells(DIR-MCFC)deterioration of catalytic activity takes place in the anode channel due to both the liquid-phase pollution and vapor-phase pollution. The liquid-phase pollution meant that catalytic activity is deteriorated by the molten salt's(62 Li2CO3/38K2CO3)adhering to the catalyst. It can be solved by installing the protective barrier in the pollutant pathway. On the other hand, the vapor-phase pollution meant that that catalytic activity is deteriorated by KOH adhering to the catalyst. Because the vapor-phase pollution is caused in the entire electrode, an effective defense method has not established yet. Moreover, a reactivation method of vapor-phase polluted catalyst has not been developed yet. In order to study the reactivation method, the adhesion form of potassium compounds in the polluted catalyst under the various gas conditions was evaluated by using a thermogravimertic analyzer in which water vapor can feed. Additionally, the activity of catalyst treated demonstratively was also tested by a differential reactor. As a result, KOH changes to K2CO3 under the condition which CO2 concentration is larger than 25%. The catalyst with K2CO3 cannot reactive. However, the activity of polluted catalyst is revived until 80% of initial activity by controlling the gas species concentration, especially for CO2. Based on the results obtained by these fundamental experiments, the reactivation methods of polluted catalyst are proposed as follows; i)Catalyst should load more in the upstream in the anode. ii)In order to reactive the polluted catalyst, the ratio of H2O to CH4 in the fuel should increase, when DIR-MCFC is under operation. iii)Gas compositions under cell maintenance mode should be applied in the case that power generation quits.