In an addition-amount controller for an exhaust gas purifying agent to be used for an exhaust emission control system of an engine, one mode is executed based on satisfaction of an execution condition for each mode, from among a plurality of control modes. The control modes includes a purification control mode in which an addition amount of NH3 or an additive serving as a generating source of the NH3 is determined according to a predetermined parameter associated with an amount of NOx in the exhaust gas, and a storage control mode in which the addition amount is set to be larger than that in the purification control mode. When a rotation speed of an output shaft of the engine is determined to be decelerated from a higher speed state than an allowable level, the execution condition of the storage control mode is satisfied after a predetermined dissatisfaction time period from start of the deceleration.