An exhaust emission control system of an internal combustion engine desorbs SOx by reversing a flow of an exhaust gas through an NOx storage-reduction catalyst, of which a structure is simplified as follows. A first exhaust pipe connected to an engine is connected to a first port of an emission switching valve having four ports. A second exhaust pipe 10, through which the exhaust gas is discharged into the atmospheric air, is connected to a second port, a third exhaust pipe connected to an inlet of a catalytic converter is connected to a third port. A fourth exhaust pipe connected to an outlet of the catalytic converter 30 is connected to a fourth port. When the emission switching valve is set in a forward flow position, the first exhaust pipe is connected to the third exhaust pipe, and the second exhaust pipe is connected to the fourth exhaust pipe, whereby the exhaust gas flows toward the outlet from the inlet within the catalytic converter. When the emission switching valve is set in a backward flow position, the first exhaust pipe is connected to the fourth exhaust pipe, and the second exhaust pipe is connected to the third exhaust pipe, whereby the exhaust gas flows toward the inlet from the outlet within the catalytic converter.