A purification controller 12 is configured to estimate the amount of ammonia occluded (i.e. ammonia occlusion amount) in an SCR catalyst 4, which is arranged in an exhaust pipe 52 of a diesel engine 51 so as to purify NOx in an exhaust gas of the diesel engine 51. More specifically, the purification controller 12 acquires upstream NO concentration data, upstream NO2 concentration data, downstream NOx concentration data, downstream NO2 concentration data and downstream ammonia concentration data. The purification controller 12 further acquires an urea injection amount. Then, the purification controller 12 estimates the ammonia occlusion amount based on the acquired upstream NO concentration data, upstream NO2 concentration data, downstream NOx concentration data, downstream NO2 concentration data and downstream ammonia concentration data, the acquired urea injection amount and reaction formulas for reduction of NOx with the SCR catalyst 4.