The emission regulations for diesel engines are continually becoming stricter to reduce pollution and conserve energy. To meet these increasingly stringent regulations, a new exhaust after-treatment device is needed. Recently, the authors proposed the simultaneous electrochemical reduction (ECR) system for diesel particulate matter (PM) and NOx. In this method, a gas-permeable electrochemical cell with a porous solid oxide electrolyte is used for PM filtering on the anode. Alkaline earth metal is coated on the cathode for NOx storage. Application of voltage to both electrodes enables the simultaneous reduction of PM and NOx by the forced flow of oxygen ions from the cathode to the anode (oxygen pumping).In this study, the basic characteristics of the ECR system were investigated, and a disk-shaped electrochemical cell was evaluated. In addition, the chassis dynamometer test cycle for the JC08 mode (post new long-term regulation in Japan) was conducted to evaluate the practicality of a honeycomb electrochemical cell. Currently, the fabrication of the honeycomb cell requires further investigation. However, the overall results show that the ECR system has the potential to simultaneously reduce more than 90% of both NOx and PM produced by diesel engines.