A power generation system comprising: a fuel cell unit including a fuel cell ; a housing accommodating the fuel cell unit ; a ventilator disposed in the housing and blowing air out of the housing ; a ventilation passage disposed in the housing , the ventilation passage allowing passage of the air blown by the ventilator and an exhaust gas discharged from the fuel cell unit ; and a discharge passage that lets the air and the exhaust gas that have passed through the ventilation passage into an atmosphere, the discharge passage being disposed outside the housing and connecting with the ventilation passage , wherein an air outlet of the ventilator is located at a higher level than a bottom face of the ventilation passage that is contiguous with the air outlet , characterized in that: the ventilation passage includes a projection , the projection projecting inward from a wall surface that defines one of the bottom face and a side face of the ventilation passage around the projection ; a drain outlet in one of the bottom face of the ventilation passage and a wall surface connecting with the bottom face , the drain outlet being located at a lower level than a merging portion where the exhaust gas and the air merge and a boundary between the ventilation passage and the discharge passage ; the bottom face of the ventilation passage slopes down toward the drain outlet ; and the air outlet is located at a leading end of the projection . a fuel cell unit including a fuel cell ; a housing accommodating the fuel cell unit ; a ventilator disposed in the housing and blowing air out of the housing ; a ventilation passage disposed in the housing , the ventilation passage allowing passage of the air blown by the ventilator and an exhaust gas discharged from the fuel cell unit ; and a discharge passage that lets the air and the exhaust gas that have passed through the ventilation passage into an atmosphere, the discharge passage being disposed outside the housing and connecting with the ventilation passage , wherein an air outlet of the ventilator is located at a higher level than a bottom face of the ventilation passage that is contiguous with the air outlet , characterized in that: the ventilation passage includes a projection , the projection projecting inward from a wall surface that defines one of the bottom face and a side face of the ventilation passage around the projection ; a drain outlet in one of the bottom face of the ventilation passage and a wall surface connecting with the bottom face , the drain outlet being located at a lower level than a merging portion where the exhaust gas and the air merge and a boundary between the ventilation passage and the discharge passage ; the bottom face of the ventilation passage slopes down toward the drain outlet ; and the air outlet is located at a leading end of the projection . the ventilation passage includes a projection , the projection projecting inward from a wall surface that defines one of the bottom face and a side face of the ventilation passage around the projection ; a drain outlet in one of the bottom face of the ventilation passage and a wall surface connecting with the bottom face , the drain outlet being located at a lower level than a merging portion where the exhaust gas and the air merge and a boundary between the ventilation passage and the discharge passage ; the bottom face of the ventilation passage slopes down toward the drain outlet ; and the air outlet is located at a leading end of the projection . <EOS>