An object of the invention is to provide a technique for improving the efficiency of performance regeneration process that is executed on NOx storage reduction catalysts provided in a plurality of branch passages branching from the exhaust passage and reduce emissions of NOx from an internal combustion engine to the atmosphere. When the quantity of NOx discharged from an internal combustion engine and flowing into an exhaust pipe is smaller than a predetermined quantity, fuel serving as reducing agent is added through a fuel addition valve while torque change moderating process is executed (from T0 to T1 ). After that, a first flow rate control valve is closed to make the flow rate of the exhaust gas flowing in the first branch passage smaller than that in the second branch passage (from T1 to T3).