An electrochemical reaction unit cell including an electrolyte layer containing a solid oxide; a cathode and an anode which face each other in a first direction with the electrolyte layer intervening therebetween; and an intermediate layer disposed between the electrolyte layer and the cathode and containing a first cerium oxide. In the electrochemical reaction unit cell, the cathode includes an active layer containing a strontium-containing perovskite oxide, a second cerium oxide, sulfur, and strontium sulfate and having ion conductivity and electron conductivity, and a grain of the strontium sulfate covers at least a portion of the surface of a grain of the second cerium oxide.