An exhaust gas treatment device for preventing a large quantity of unburned HC components from being released to the outside until a diesel oxidation catalyst reaches an activation temperature after early post-injection. By reducing a throttle opening of a variable throttle mechanism (77) provided in an oil passage (72), or increasing a set value of an oil pressure level detected in an oil pressure sensor (78) provided on the downstream side of the variable throttle mechanism, power of an oil circulation pump (74) is increased before a start point t1 of the early post-injection. With this arrangement, by increasing a load of a diesel engine to increase a temperature rise gradient of exhaust gas at the stage of increasing the temperature of the diesel oxidation catalyst, the unburned HC component released until the diesel oxidation catalyst reaches the activation temperature is reduced.