An exhaust gas control apparatus for an internal combustion engine, in which a NOx selective reduction catalyst is disposed in an engine exhaust gas passage, ammonia is supplied to the NOx selective reduction catalyst to cause the NOx selective reduction catalyst to adsorb the ammonia, and NOx in exhaust gas is selectively reduced using the ammonia adsorbed, the exhaust gas control apparatus being characterized in that: as states of adsorption of the ammonia by the NOx selective reduction catalyst, there are a first adsorption state of the ammonia that occurs if the ammonia is adsorbed by the NOx selective reduction catalyst while a temperature of the NOx selective reduction catalyst is in a lower side temperature range, and an amount of desorption of the ammonia in which state is maximized in the lower side temperature range as the temperature of the NOx selective reduction catalyst increases, and a second adsorption state of the ammonia that occurs if the ammonia is adsorbed or has already been adsorbed by the NOx selective reduction catalyst while the temperature of the NOx selective reduction catalyst is in a higher side temperature range higher than the lower side temperature range, and an amount of desorption of the ammonia in which state is maximized in the higher side temperature range as the temperature of the NOx selective reduction catalyst increases; and the exhaust gas control apparatus comprises a controller for estimating an amount of adsorbed ammonia that is in the first adsorption state to control the amount of adsorbed ammonia that is in the first adsorption state, and for estimating an amount of adsorbed ammonia that is in the second adsorption state to control the amount of adsorbed ammonia that is in the second adsorption state, with the use of at least relations, stored in advance, between the temperature and a proportion of the amount of ammonia that is adsorbed in the first adsorption state and a proportion of the amount of ammonia that is adsorbed in the second adsorption state. as states of adsorption of the ammonia by the NOx selective reduction catalyst, there are a first adsorption state of the ammonia that occurs if the ammonia is adsorbed by the NOx selective reduction catalyst while a temperature of the NOx selective reduction catalyst is in a lower side temperature range, and an amount of desorption of the ammonia in which state is maximized in the lower side temperature range as the temperature of the NOx selective reduction catalyst increases, and a second adsorption state of the ammonia that occurs if the ammonia is adsorbed or has already been adsorbed by the NOx selective reduction catalyst while the temperature of the NOx selective reduction catalyst is in a higher side temperature range higher than the lower side temperature range, and an amount of desorption of the ammonia in which state is maximized in the higher side temperature range as the temperature of the NOx selective reduction catalyst increases; and the exhaust gas control apparatus comprises a controller for estimating an amount of adsorbed ammonia that is in the first adsorption state to control the amount of adsorbed ammonia that is in the first adsorption state, and for estimating an amount of adsorbed ammonia that is in the second adsorption state to control the amount of adsorbed ammonia that is in the second adsorption state, with the use of at least relations, stored in advance, between the temperature and a proportion of the amount of ammonia that is adsorbed in the first adsorption state and a proportion of the amount of ammonia that is adsorbed in the second adsorption state.