An exhaust emission control apparatus that adds additive solution to exhaust gas from an on-board internal combustion engine in order to purify the exhaust gas, the exhaust emission control apparatus comprises: a level sensor provided in a tank in which the additive solution is stored, the level sensor being configured to detect a liquid surface level that indicates the height of a liquid surface of the additive solution; a controller configured to (i) calculate a remaining amount of the additive solution inside the tank on the basis of the liquid surface level detected by the level sensor, (ii) calculate an available travel distance that a vehicle is allowed to travel while purifying exhaust gas by using the additive solution, (iii) store the calculated available travel distance, and (iv) calculate a period travel distance that is a travel distance of the vehicle from when the available travel distance is calculated to when the available travel distance is calculated next; and a readout provided in a vehicle cabin, the readout being configured to indicate the calculated available travel distance, characterized in that the controller is configured to: when the remaining amount of the additive solution exceeds a predetermined amount, execute a first calculation process in which the available travel distance is calculated with reference to the remaining amount of the additive solution, after the remaining amount of the additive solution becomes smaller than or equal to the predetermined amount, execute a second calculation process in which the available travel distance is calculated by subtracting the period travel distance from the stored previously calculated available travel distance, acquire a vehicle speed, and calculate the period travel distance on the basis of the vehicle speed at the time when the available travel distance is calculated and a calculation time interval of the available travel distance. a level sensor provided in a tank in which the additive solution is stored, the level sensor being configured to detect a liquid surface level that indicates the height of a liquid surface of the additive solution; a controller configured to (i) calculate a remaining amount of the additive solution inside the tank on the basis of the liquid surface level detected by the level sensor, (ii) calculate an available travel distance that a vehicle is allowed to travel while purifying exhaust gas by using the additive solution, (iii) store the calculated available travel distance, and (iv) calculate a period travel distance that is a travel distance of the vehicle from when the available travel distance is calculated to when the available travel distance is calculated next; and (i) calculate a remaining amount of the additive solution inside the tank on the basis of the liquid surface level detected by the level sensor, (ii) calculate an available travel distance that a vehicle is allowed to travel while purifying exhaust gas by using the additive solution, (iii) store the calculated available travel distance, and (iv) calculate a period travel distance that is a travel distance of the vehicle from when the available travel distance is calculated to when the available travel distance is calculated next; and a readout provided in a vehicle cabin, the readout being configured to indicate the calculated available travel distance, characterized in that the controller is configured to: when the remaining amount of the additive solution exceeds a predetermined amount, execute a first calculation process in which the available travel distance is calculated with reference to the remaining amount of the additive solution, after the remaining amount of the additive solution becomes smaller than or equal to the predetermined amount, execute a second calculation process in which the available travel distance is calculated by subtracting the period travel distance from the stored previously calculated available travel distance, acquire a vehicle speed, and calculate the period travel distance on the basis of the vehicle speed at the time when the available travel distance is calculated and a calculation time interval of the available travel distance. when the remaining amount of the additive solution exceeds a predetermined amount, execute a first calculation process in which the available travel distance is calculated with reference to the remaining amount of the additive solution, after the remaining amount of the additive solution becomes smaller than or equal to the predetermined amount, execute a second calculation process in which the available travel distance is calculated by subtracting the period travel distance from the stored previously calculated available travel distance, acquire a vehicle speed, and calculate the period travel distance on the basis of the vehicle speed at the time when the available travel distance is calculated and a calculation time interval of the available travel distance.