An apparatus may have a selective catalytic reduction NOx catalyst including a high-temperature catalyst layer having high capability of reducing NOx at high temperatures and a low-temperature catalyst layer having higher capability of reducing NOx at low temperatures than that of the high-temperature catalyst layer. The low-temperature catalyst layer may be arranged closer to a catalyst substrate than the high-temperature catalyst layer. A supply valve may add an addition quantity of reducing agent for reducing NOx to exhaust gas flowing into the selective catalytic reduction NOx catalyst. A controller may comprise at least one processor configured to control addition of the reducing agent by the supply valve such that the reducing agent concentration in a reducing agent atmosphere formed in the exhaust gas flowing into the selective catalytic reduction NOx catalyst is higher when the temperature of the selective catalytic reduction NOx catalyst is in a specific low temperature range.