In order to meet the reinforcement of worldwide environmental regulations, latest diesel engines for industrial machinery are required to reduce the emission of harmful gases such as carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxide (NOx), and particulate matter (PM). For this reason, some of the diesel engines are equipped with exhaust gas treatment devices such as diesel particulate filter (DPF), diesel oxidation catalyst (DOC) and selective catalytic reduction (SCR) catalyst. However, applications of such industrial diesel engines bring about excessive back pressure increase and deterioration in the performance of the catalysts when continuous operation is performed at low load conditions: soot accumulates on the inlet faces of DOC and DPF, causing face plugging issues. To resolve this issue, it is necessary for the system to be equipped with certain additional devices to raise an exhaust gas temperature to a high level enough to burn out the soot [1]. In this research, in order to solve the face plugging at the inlet of DOC without using such an additional equipment, we studied the cause of the face plugging of DOC. First, in order to estimate the cause of face plugging, we grasped the engine operating conditions leading to the face plugging and analyzed the soot/coke deposits accumulating on the inlet face of DOC under specific conditions. Next, in order to identify the origin of the face plugging causative deposits, we performed a component analysis of an engine exhaust gas and also conducted catalytic reaction experiments using a model exhaust gas. Then, verification experiments using an engine were carried out. As a result, we found that the production of less combustible HC by incomplete catalytic oxidation is one of the factors of the plugging problem of DOC.