An electrochemical reaction cell stack comprising an electrochemical reaction block including three or more electrochemical reaction units arranged in a first direction, each of the electrochemical reaction units including an electrolyte layer, a cathode and an anode which face each other in the first direction with the electrolyte layer intervening therebetween, and an anode chamber facing the anode, wherein: the three or more electrochemical reaction units include one or more upstream electrochemical reaction units, and two or more downstream electrochemical reaction units; the electrochemical reaction block includes a gas introduction flow passage for introducing a fuel gas from the outside of the electrochemical reaction block to the inside thereof, a gas discharge flow passage for discharging the fuel gas from the inside of the electrochemical reaction block to the outside thereof, and a gas transfer flow passage for transferring the fuel gas discharged from the anode chamber of the upstream electrochemical reaction unit to the downstream electrochemical reaction unit; each of the upstream electrochemical reaction units includes an upstream introduction communication passage for connecting the anode chamber on the upstream side which is formed in the upstream electrochemical reaction unit to the gas introduction flow passage, and an upstream discharge communication passage for connecting the upstream-side anode chamber to the gas transfer flow passage; and each of the downstream electrochemical reaction units includes a downstream introduction communication passage for connecting the anode chamber on the downstream side which is formed in the downstream electrochemical reaction unit to the gas transfer flow passage, and a downstream discharge communication passage for connecting the downstream-side anode chamber to the gas discharge flow passage, the electrochemical reaction cell stack being characterized in that: in each of the two or more downstream electrochemical reaction units, the total volume of the downstream introduction communication passage and the downstream discharge communication passage is smaller than the total volume of the upstream introduction communication passage and the upstream discharge communication passage; and the gas introduction flow passage, the gas transfer flow passage and the gas discharge flow passage extend in the first direction through the electrochemical reaction units. the three or more electrochemical reaction units include one or more upstream electrochemical reaction units, and two or more downstream electrochemical reaction units; the electrochemical reaction block includes a gas introduction flow passage for introducing a fuel gas from the outside of the electrochemical reaction block to the inside thereof, a gas discharge flow passage for discharging the fuel gas from the inside of the electrochemical reaction block to the outside thereof, and a gas transfer flow passage for transferring the fuel gas discharged from the anode chamber of the upstream electrochemical reaction unit to the downstream electrochemical reaction unit; each of the upstream electrochemical reaction units includes an upstream introduction communication passage for connecting the anode chamber on the upstream side which is formed in the upstream electrochemical reaction unit to the gas introduction flow passage, and an upstream discharge communication passage for connecting the upstream-side anode chamber to the gas transfer flow passage; and each of the downstream electrochemical reaction units includes a downstream introduction communication passage for connecting the anode chamber on the downstream side which is formed in the downstream electrochemical reaction unit to the gas transfer flow passage, and a downstream discharge communication passage for connecting the downstream-side anode chamber to the gas discharge flow passage, the electrochemical reaction cell stack being characterized in that: in each of the two or more downstream electrochemical reaction units, the total volume of the downstream introduction communication passage and the downstream discharge communication passage is smaller than the total volume of the upstream introduction communication passage and the upstream discharge communication passage; and the gas introduction flow passage, the gas transfer flow passage and the gas discharge flow passage extend in the first direction through the electrochemical reaction units.