A fuel cell stack, comprising: a plurality of fuel cell modules stacked in a predetermined direction, and an external circuit, each of the fuel cell modules comprising: a first membrane electrode assembly and a second membrane electrode assembly, each including an electrolyte membrane and a first electrode and a second electrode arranged to sandwich two faces of the electrolyte membrane, wherein the first membrane electrode assembly and the second membrane electrode assembly are arranged, such that the respective first electrodes are opposed to each other; a first reactive gas flow path arranged to supply a first reactive gas to the first electrodes included in the first membrane electrode assembly and the second membrane electrode assembly; a second reactive gas flow path arranged to supply a second reactive gas to the second electrodes included in the first membrane electrode assembly and the second membrane electrode assembly; and a coolant flow path, wherein the external circuit is arranged to electrically connect the first membrane electrode assembly and the second membrane electrode assembly in parallel with each other, characterized in that the coolant flow path is arranged to cool down the second electrodes included in the first membrane electrode assembly and the second membrane electrode assembly, the fuel cell stack does not have a coolant flow path for cooling down the first electrodes, but has a first flow path-forming member located between the first membrane electrode assembly and the second membrane electrode assembly to define the first reactive gas flow path, an insulator is located between an adjacent pair of the fuel cell modules, the coolant flow path is formed inside the insulator, and the plurality of fuel cell modules are connected in series. a plurality of fuel cell modules stacked in a predetermined direction, and an external circuit, each of the fuel cell modules comprising: a first membrane electrode assembly and a second membrane electrode assembly, each including an electrolyte membrane and a first electrode and a second electrode arranged to sandwich two faces of the electrolyte membrane, wherein the first membrane electrode assembly and the second membrane electrode assembly are arranged, such that the respective first electrodes are opposed to each other; a first reactive gas flow path arranged to supply a first reactive gas to the first electrodes included in the first membrane electrode assembly and the second membrane electrode assembly; a second reactive gas flow path arranged to supply a second reactive gas to the second electrodes included in the first membrane electrode assembly and the second membrane electrode assembly; and a coolant flow path, wherein the external circuit is arranged to electrically connect the first membrane electrode assembly and the second membrane electrode assembly in parallel with each other, characterized in that the coolant flow path is arranged to cool down the second electrodes included in the first membrane electrode assembly and the second membrane electrode assembly, the fuel cell stack does not have a coolant flow path for cooling down the first electrodes, but has a first flow path-forming member located between the first membrane electrode assembly and the second membrane electrode assembly to define the first reactive gas flow path, an insulator is located between an adjacent pair of the fuel cell modules, the coolant flow path is formed inside the insulator, and the plurality of fuel cell modules are connected in series. a first membrane electrode assembly and a second membrane electrode assembly, each including an electrolyte membrane and a first electrode and a second electrode arranged to sandwich two faces of the electrolyte membrane, wherein the first membrane electrode assembly and the second membrane electrode assembly are arranged, such that the respective first electrodes are opposed to each other; a first reactive gas flow path arranged to supply a first reactive gas to the first electrodes included in the first membrane electrode assembly and the second membrane electrode assembly; a second reactive gas flow path arranged to supply a second reactive gas to the second electrodes included in the first membrane electrode assembly and the second membrane electrode assembly; and a coolant flow path, wherein the external circuit is arranged to electrically connect the first membrane electrode assembly and the second membrane electrode assembly in parallel with each other, characterized in that the coolant flow path is arranged to cool down the second electrodes included in the first membrane electrode assembly and the second membrane electrode assembly, the fuel cell stack does not have a coolant flow path for cooling down the first electrodes, but has a first flow path-forming member located between the first membrane electrode assembly and the second membrane electrode assembly to define the first reactive gas flow path, an insulator is located between an adjacent pair of the fuel cell modules, the coolant flow path is formed inside the insulator, and the plurality of fuel cell modules are connected in series.