into the ammonia adsorbing tower 5 via a damper d 5 in this ammonia adsorbing tower 5 ammonia adsorbed by the ammonia adsorbent is desorbed and then returned to an upstream side of the denitrating device 4 of the exhaust gas boiler 2 through the recovery gas duct 13 the dampers d 1 to d 6 are switched suitably in accordance with the adsorbing state of ammonia or when a certain time has passed so that these ammonia adsorbing towers 5 6 are alternated to adsorb or desorb ammonia an ammonia adsorbent comprising 75 of titania 5 of vanadium pentoxide and a molding auxiliary agent was molded into pellets having a diameter of 3 mm and a combustion exhaust gas containing 20 ppm of ammonia at a gas temperature of 100 u00b0 c was then introduced into the pellets to adsorb ammonia sufficiently afterward the feed of the combustion exhaust gas was stopped and hot air having a gas temperature adjusted to 100 u00b0 c was introduced thereinto at this time an ammonia concentration change with time at the outlet of the ammonia adsorbent was measured furthermore changes with time at air temperatures of 200 u00b0 c 300 u00b0 c and 400 u00b0 c were measured in the same manner and the results are shown in table 1 table 1______________________________________ammonia concentrations at outlet of adsorbentgastemp after after after after after u00b0 c 10 min 20 min 30 min 60 min 120 min______________________________________100 2 3 ppm 2 3 ppm 2 3 ppm 2 3 ppm 2 3 ppm200 3 5 ppm 5 10 ppm 5 10 ppm 5 10 ppm 5 10 ppm300 60 ppm 100 ppm 100 ppm 50 ppm 10 ppm400 120 ppm 180 ppm 140 ppm 20 ppm.