A power generation system comprising: a fuel cell system including a fuel cell configured to generate electric power using a fuel gas and an oxidizing gas, a case configured to house the fuel cell, a reformer configured to generate the fuel gas from a raw material and steam, and a combustor configured to heat the reformer; an oxidizing gas supply unit configured to supply the oxidizing gas to the fuel cell; a ventilator; a controller; a combustion device; a discharge passage formed to cause the case and an exhaust port of the combustion device to communicate with each other and configured to discharge an exhaust gas from the fuel cell system and an exhaust gas from the combustion device to an atmosphere through an opening of the discharge passage, the opening being open to the atmosphere, and an operation detector configured to detect that the combustion device has operated, wherein: the ventilator is configured to discharge a gas in the case to the discharge passage to ventilate an inside of the case; and characterized in that the controller is configured to cause the combustion device to operate when the ventilator is in a stop state and then cause the ventilator to operate when the fuel cell system is activated, and wherein in a case where the operation detector detects that the combustion device has operated when the ventilator is in a stop state, the controller is configured to cause the ventilator to operate when the fuel cell system is activated. a fuel cell system including a fuel cell configured to generate electric power using a fuel gas and an oxidizing gas, a case configured to house the fuel cell, a reformer configured to generate the fuel gas from a raw material and steam, and a combustor configured to heat the reformer; an oxidizing gas supply unit configured to supply the oxidizing gas to the fuel cell; a ventilator; a controller; a combustion device; a discharge passage formed to cause the case and an exhaust port of the combustion device to communicate with each other and configured to discharge an exhaust gas from the fuel cell system and an exhaust gas from the combustion device to an atmosphere through an opening of the discharge passage, the opening being open to the atmosphere, and an operation detector configured to detect that the combustion device has operated, wherein: the ventilator is configured to discharge a gas in the case to the discharge passage to ventilate an inside of the case; and characterized in that the controller is configured to cause the combustion device to operate when the ventilator is in a stop state and then cause the ventilator to operate when the fuel cell system is activated, and wherein in a case where the operation detector detects that the combustion device has operated when the ventilator is in a stop state, the controller is configured to cause the ventilator to operate when the fuel cell system is activated. the ventilator is configured to discharge a gas in the case to the discharge passage to ventilate an inside of the case; and characterized in that the controller is configured to cause the combustion device to operate when the ventilator is in a stop state and then cause the ventilator to operate when the fuel cell system is activated, and wherein in a case where the operation detector detects that the combustion device has operated when the ventilator is in a stop state, the controller is configured to cause the ventilator to operate when the fuel cell system is activated.