In a fuel cells power generation system provided with a power generation module having a plurality of fuel cells, the structure is made such that a cross sectional area of at least one of a fuel flow path and an air flow path is larger in an inner portion of the power generation module and smaller in an outer portion thereof. Accordingly, gas tends to flow through the inner portion of the power generation module, a gas flow rate is quickened, and it is possible to uniformize a molar flow rate of the fuel and the air supplied to the fuel cell, even in a state in which a temperature distribution of the module is not uniform within the power generation module.