An exhaust gas control apparatus includes a PM trapping filter provided in an exhaust gas passage of an internal combustion engine, an oxygen concentration sensor provided upstream of the filter, a device that estimates the air-fuel ratio of exhaust gas supplied to the oxygen concentration sensor, and a device that estimates the amount of PM trapped in the filter based on the estimated exhaust gas air-fuel ratio and the oxygen concentration sensor output. The amount of PM trapped in the filter is estimated using the fact that the oxygen concentration sensor output value changes according to the exhaust gas air-fuel ratio and the amount of PM accumulated on the oxygen concentration sensor. As a result, the structure of the exhaust passage is able to be kept simple because only an oxygen concentration sensor is added upstream of the filter.