An exhaust purifying system comprising: a selective catalytic reduction that is provided in an exhaust passage of an internal combustion engine to purify NOx contained in an exhaust gas configured to use, as a reducing agent, ammonia produced from urea water; an injection unit configured to inject the urea water into a portion of the exhaust passage which is upstream from the selective catalytic reduction; a calculation unit configured to calculate an estimated adsorption amount of the ammonia adsorbed in the selective catalytic reduction; an injection control unit configured to execute an injection control of the urea water injection unit based on the estimated adsorption amount; a change unit configured to change the estimated adsorption amount used for the injection control into a value increased or decreased by a predetermined amount when a predetermined condition is established that can cause a difference between an actual adsorption amount of the ammonia adsorbed in the selective catalytic reduction and the estimated adsorption amount; a determination unit configured to determine whether an NOx purification rate of the selective catalytic reduction is decreased due to the increase or decrease of the estimated adsorption amount; and a maintaining unit configured to maintain the value increased or decreased by the change unit when the determination unit determines that the NOx purification rate is not decreased, characterised by a correction unit configured to return the value changed by the change unit to the estimated adsorption amount of the ammonia calculated by the calculation unit before the change unit changes the estimated adsorption amount when the determination unit determines that the NOx purification rate has been decreased. a selective catalytic reduction that is provided in an exhaust passage of an internal combustion engine to purify NOx contained in an exhaust gas configured to use, as a reducing agent, ammonia produced from urea water; an injection unit configured to inject the urea water into a portion of the exhaust passage which is upstream from the selective catalytic reduction; a calculation unit configured to calculate an estimated adsorption amount of the ammonia adsorbed in the selective catalytic reduction; an injection control unit configured to execute an injection control of the urea water injection unit based on the estimated adsorption amount; a change unit configured to change the estimated adsorption amount used for the injection control into a value increased or decreased by a predetermined amount when a predetermined condition is established that can cause a difference between an actual adsorption amount of the ammonia adsorbed in the selective catalytic reduction and the estimated adsorption amount; a determination unit configured to determine whether an NOx purification rate of the selective catalytic reduction is decreased due to the increase or decrease of the estimated adsorption amount; and a maintaining unit configured to maintain the value increased or decreased by the change unit when the determination unit determines that the NOx purification rate is not decreased, characterised by a correction unit configured to return the value changed by the change unit to the estimated adsorption amount of the ammonia calculated by the calculation unit before the change unit changes the estimated adsorption amount when the determination unit determines that the NOx purification rate has been decreased.