A regeneration apparatus comprises a heater 2 which heats a catalyst 1 at the time of stopping fuel supply to an engine, a pump 4 which supplies an oxidizing gas or a reducing gas to the heated catalyst 1 and a switching valve 3. In the oxidizing step, catalytic poisons, for example hydrocarbon, which are absorbed on a noble metal are oxidized and removed. The particles of the noble metal become oxidized particles as pure oxide and the like. After the oxidizing step, the reducing step is carried out. In the reducing step, particles of the noble metal are reduced and re-dispersed on the support simultaneously and as a result, the particles of the noble metal become fine. Thus, a deteriorated catalyst can be regenerated without interrupting the use of catalyst and also without deteriorating the drivability.