An exhaust gas after-treatment device for a diesel engine is provided. The exhaust gas after-treatment device comprises an oxidation treatment unit having a diesel oxidation catalyst (DOC), an adsorption treatment unit having a diesel particulate filter (DPF), and a discharge unit for discharging exhaust gas that is filtration-treated through the oxidation treatment unit and the adsorption treatment unit, wherein the DOC of the oxidation treatment unit is provided with a baffle on the front thereof for even diffusion-introduction of exhaust gas.