PURPOSE: To prevent increase of growth of sulphate at the time when exhaust gas temperature is high with an oxidization catalyzer to purify exhaust gas of a diesel engine.

CONSTITUTION: This exhaust emission control device 1 has a first case 10 enclosing an oxidization catalyzer 20 and a second case 30 covering the first case 10, and between the first case 10 and the second case 30, a by-pass passage 46 is formed. An entrance 40 of exhaust gas is branched into a first passage communicating through to the by-pass passage 46, and a first change-over valve 70 and a second change-over valve 72 are respectively provided in each of the passages. A controller 60 opens and closes the change-over valves in accordance with information of a sensor 64 to detect temperature of exhaust gas G and when temperature is lower than a set value, it flows exhaust gas to a flow passage F1 and when it becomes higher than the set value, it flows exhaust gas to a flow passage F2.