An exhaust emission control apparatus is provided with a supply device, a catalyst, and a gas pressure reduction part. The supply device supplies a reducing agent to an exhaust passage. The catalyst purifies an exhaust gas by the use of the reducing agent. The gas pressure reduction part can make a gas pressure near the supply port lower than the gas pressure on the inside of a supply device body. A NOx catalyst adsorbs nitrogen oxide contained in the exhaust. NOx adsorbed by the NOx catalyst is desorbed from the NOx catalyst when the exhaust gas is purified. An ECU estimates an adsorption amount of NOx. Then, the ECU estimates a desorption amount of NOx desorbed from the NOx catalyst on the basis of the estimated adsorption amount of NOx.