PURPOSE: To eliminate the unnecessary regeneration cost or the destruction of a catalyst due to the meaningless extension of regeneration, by determining a catalyst regeneration finish period by using the concns. of dust before and after a catalyst packed bed and accurately judging the final point of catalyst regeneration effect to restore a reaction ratio to the max. degree.

CONSTITUTION: For example, the concns. of dust observed by dust meters 22, 23 arranged before and behind the reactor (catalyst bed) 17 in the waste gas denitration equipment 21 of a coke oven 1 are set to D1, D2. When the regeneration of a catalyst is started, the dust accumulated to the catalyst bed 17 is carried away by passing gas and D2 abruptly increases and subsequently reduces gradually as accumulated dust is reduced. The point of time when D1=D2 is formed is set as the final point of regeneration effect to finish the regeneration of the catalyst. After the finish of regeneration, because a part of dust is accumulated on the catalyst bed 17 asin a usual state before regeneration, D2 becomes slightly lower than D1.