The present invention relates to an exhaust gas treatment device of a diesel engine that is capable of increasing the concentration of PM in the EGR gas, wherein an exhaust gas separator 2 has electrodes 12 and 13 having different polarities from each other to allow the PM in the exhaust gas 3 to be charged to a given polarity by means of corona discharge between the electrodes 12 and 13 and thus to permit a peripheral wall 14 surrounding the exhaust gas swirl chamber 9 to have the electrode 13 having the opposite polarity to the charged PM, and the charged PM in the exhaust gas 3 swirling the exhaust gas swirl chamber 9 is localizedly positioned around the peripheral wall 14 of the exhaust gas swirl chamber 9 through a centrifugal force and an electrostatic force to allow the exhaust gas 3 containing the localizedly positioned PM to be separated as the EGR gas 4 and sent to a terminal end portion 15 of the exhaust gas swirl chamber 9, while the exhaust gas 3 around the central cylinder 7 is being separated as the emission gas 5 and sent to the interior of the central cylinder 7 through the plurality of emission gas entry holes 8.