An electrochemical reactor is arranged inside an exhaust passage of an internal combustion engine and is provided with a plurality of groups of cells. Each group of cell has a plurality of cells, each cell has an ion conducting solid electrolyte layer, and an anode layer and cathode layer arranged on a surface of the solid electrolyte layer. Each group of cells is configured so that all of the exhaust gas flows into passages defined by cells configuring the group of cells and so that both of the anode layers and the cathode layers are exposed to each passage. The plurality of groups of cells are arranged aligned in a direction of flow of exhaust gas and different groups of cells are connected to a power source in parallel with each other.