An exhaust emission control device reduces nitrogen oxide in exhaust air from an engine. The device includes an exhaust pipe, a catalyst, a supply device, a NOx detecting device, and a control device. The pipe defines a passage for the exhaust air. The catalyst is disposed in the pipe and promotes reduction reaction of the nitrogen oxide. The supply device supplies an additive agent, which is used for the reaction, to an upstream side of the catalyst in the exhaust air flow direction. The detecting device detects the nitrogen oxide included in the exhaust airs which has passed through the catalyst. The control device controls the supply device. The control device determines a deterioration state of the catalyst based on a result detected by the detecting device and reduces an amount of the additive agent, which is supplied, according to the determined deterioration state.