Exhaust gas control apparatus and method for internal combustion engine

In an exhaust passage (2) of an engine (1), there are provided an upstream side NOx storage reduction catalyst (7a); a downstream side NOx storage reduction catalyst (7b); an upstream side NOx sensor (29a) and a downstream side NOx sensor (29b) each of which detects a NOx concentration in exhaust gas in an outlet of the catalyst; and an upstream side air-fuel ratio sensor (31a) and a downstream side air-fuel ratio sensor (31b) each of which detects an air-fuel ratio of the exhaust gas in the outlet of the catalyst. An electronic control unit (ECU) 30 starts a rich spike operation when an output from the NOx sensor reaches a predetermined value, and ends the rich spike operation when an output from the air-fuel ratio becomes a predetermined rich air-fuel ratio. The ECU (30) decides start time and end time of the rich spike operation using the upstream side NOx sensor (29a) and the upstream side air-fuel ratio sensor, when the engine is being operated at high load. The ECU decides the start time and the end time of the rich spike operation using the downstream side NOx sensor (29b) and the downstream side air-fuel ratio sensor, when the engine is being operated at low or medium load.