To provide an exhaust emission control device of a diesel engine which is reduced in white smoke even immediately after a low temperature start and can prevent black smoke.

The exhaust emission control device of a diesel engine comprising an exhaust emission control device (5) installed in a exhaust emission system (4), a bypass passage (6) bypassing the exhaust emission control device, and a bypass valve (7) installed in the bypass passage has a control means (9). The exhaust emission control device is provided with a catalyst (5f), an exhaust emission heating means (5h) at the upstream of the catalyst and an exhaust emission temperature detecting means (TL) at the downstream of the catalyst for detecting a low exhaust emission temperature, and provided with an engine load detecting means (Sc). The control means decreases an exhaust emission flow into the emission control device when the temperature at the downstream of the catalyst is not higher than the predetermined value based on the information of the exhaust emission temperature detecting means, starts a catalyst heating operation after opening the bypass valve when the engine load is not higher than a predetermined value based on information by the engine load detecting means, and starts a catalyst overheating operation without opening the bypass valve when the engine load exceeds a predetermined valve.