A power system comprising: a fuel cell system that has a fuel cell for generating power by using fuel gas and oxidant gas and a housing for accommodating the fuel cell; a controller; a combustion apparatus; a discharge passage that is disposed so as to cause the housing and an air outlet of the combustion apparatus to communicate with each other, and that discharges exhaust gas discharged from the fuel cell system and exhaust gas discharged from the combustion apparatus to an atmosphere; an air discharger for discharging air inside the housing of the fuel cell system into the discharge passage; a ventilation passage that connects the air discharger and the discharge passage to each other for the air discharger to discharge the air; a reverse flow prevention device that is disposed in the discharge passage or the ventilation passage, on an upstream side from a merging site where the exhaust gas discharged from the fuel cell system and the exhaust gas discharged from the combustion apparatus merge together, in the discharge passage; and a combustible gas detection sensor that is disposed inside the housing so as to detect combustible gas present inside the housing, characterized in that the controller operates the air discharger, when it is confirmed that the air discharger is stopped and when the combustible gas detection sensor continuously detects the combustible gas having first concentration or higher concentration during a first period of time, wherein in a state where the air discharger is operated, the controller determines that the power system is abnormal, or gives a notification of the abnormality, when the combustible gas detection sensor continuously detects the combustible gas having second concentration or higher concentration during a second period of time, wherein in the state where the air discharger is operated, the controller performs a stop operation for stopping the air discharger (i) after a predetermined period of time elapses or (ii) when the combustible gas detection sensor continuously detects the combustible gas having third concentration or lower concentration during a third period of time, and wherein after the stop operation is repeatedly performed a predetermined number of times, the controller determines that the power system is abnormal, or gives a notification of the abnormality. a fuel cell system that has a fuel cell for generating power by using fuel gas and oxidant gas and a housing for accommodating the fuel cell; a controller; a combustion apparatus; a discharge passage that is disposed so as to cause the housing and an air outlet of the combustion apparatus to communicate with each other, and that discharges exhaust gas discharged from the fuel cell system and exhaust gas discharged from the combustion apparatus to an atmosphere; an air discharger for discharging air inside the housing of the fuel cell system into the discharge passage; a ventilation passage that connects the air discharger and the discharge passage to each other for the air discharger to discharge the air; a reverse flow prevention device that is disposed in the discharge passage or the ventilation passage, on an upstream side from a merging site where the exhaust gas discharged from the fuel cell system and the exhaust gas discharged from the combustion apparatus merge together, in the discharge passage; and a combustible gas detection sensor that is disposed inside the housing so as to detect combustible gas present inside the housing, characterized in that the controller operates the air discharger, when it is confirmed that the air discharger is stopped and when the combustible gas detection sensor continuously detects the combustible gas having first concentration or higher concentration during a first period of time, wherein in a state where the air discharger is operated, the controller determines that the power system is abnormal, or gives a notification of the abnormality, when the combustible gas detection sensor continuously detects the combustible gas having second concentration or higher concentration during a second period of time, wherein in the state where the air discharger is operated, the controller performs a stop operation for stopping the air discharger (i) after a predetermined period of time elapses or (ii) when the combustible gas detection sensor continuously detects the combustible gas having third concentration or lower concentration during a third period of time, and wherein after the stop operation is repeatedly performed a predetermined number of times, the controller determines that the power system is abnormal, or gives a notification of the abnormality.