An exhaust purification system comprising: an exhaust after-treatment apparatus which is provided on an exhaust passage of an internal combustion engine and which includes a catalyst (31, 32) for purifying exhaust gas discharged from the internal combustion engine; catalyst temperature retention control means for executing a catalyst temperature retention control for reducing an intake air flow to suppress a reduction in temperature of the catalyst (31, 32) when the internal combustion engine is in a motoring state where fuel injection into the internal combustion engine is stopped; prohibition means for prohibiting the execution of the catalyst temperature retention control in a case where an activation of an exhaust brake system which is connected to the internal combustion engine is detected while the internal combustion engine is in the motoring state; discharge amount storage means for acquiring and storing in advance a carbon monoxide amount which is discharged from the internal combustion engine when the catalyst temperature retention control is executed while the internal combustion engine is in an idling state; calorific value estimation means for estimating a carbon monoxide calorific value based on the carbon monoxide amount which is read out from the discharge amount storage means when the catalyst temperature retention control is executed while the internal combustion engine is in the idling state; and catalyst temperature estimation means for estimating a temperature of the catalyst based on the carbon monoxide calorific value which is estimated by the calorific value estimation means when the catalyst temperature retention control is executed while the internal combustion engine is in the idling state. an exhaust after-treatment apparatus which is provided on an exhaust passage of an internal combustion engine and which includes a catalyst (31, 32) for purifying exhaust gas discharged from the internal combustion engine; catalyst temperature retention control means for executing a catalyst temperature retention control for reducing an intake air flow to suppress a reduction in temperature of the catalyst (31, 32) when the internal combustion engine is in a motoring state where fuel injection into the internal combustion engine is stopped; prohibition means for prohibiting the execution of the catalyst temperature retention control in a case where an activation of an exhaust brake system which is connected to the internal combustion engine is detected while the internal combustion engine is in the motoring state; discharge amount storage means for acquiring and storing in advance a carbon monoxide amount which is discharged from the internal combustion engine when the catalyst temperature retention control is executed while the internal combustion engine is in an idling state; calorific value estimation means for estimating a carbon monoxide calorific value based on the carbon monoxide amount which is read out from the discharge amount storage means when the catalyst temperature retention control is executed while the internal combustion engine is in the idling state; and catalyst temperature estimation means for estimating a temperature of the catalyst based on the carbon monoxide calorific value which is estimated by the calorific value estimation means when the catalyst temperature retention control is executed while the internal combustion engine is in the idling state.