A power generation system comprising: an air intake passage; 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, and an air supply unit configured to supply external air to the case through the air intake passage; and an air intake temperature detector configured to detect a temperature of the intake air supplied to the case; characterized by a combustion device provided outside of the case and including a combustor configured to combust a fuel to generate an amount of heat such that the generated heat is used to heat water; an exhaust gas passage including a first passage extending from an upstream end connected to the fuel cell system and configured to discharge an exhaust gas from the fuel cell system, a second passage extending from an upstream end connected to the combustion device and configured to discharge a flue gas generated in the combustion device to the outside, a joining portion in which both of the passages are joined, and a common passage extending from the joining portion to a downstream end opened to atmosphere, and configured to discharge the exhaust gas from the fuel cell system, and the flue gas generated in the combustion device to the outside; and a controller, wherein: the air intake passage and the exhaust gas passage are provided such that the air intake passage makes contact with at least a portion of the exhaust gas passage so as to allow heat exchange to occur between media flowing through the passages, and the controller is configured to cause the air intake temperature detector to detect the temperature of the intake air supplied to the case when a power generation command is input to the power generation system, and cause the combustion device to activate when the fuel cell system is activated and the temperature detected by the air intake temperature detector is equal to or lower than a first predetermined temperature. an air intake passage; 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, and an air supply unit configured to supply external air to the case through the air intake passage; and an air intake temperature detector configured to detect a temperature of the intake air supplied to the case; characterized by a combustion device provided outside of the case and including a combustor configured to combust a fuel to generate an amount of heat such that the generated heat is used to heat water; an exhaust gas passage including a first passage extending from an upstream end connected to the fuel cell system and configured to discharge an exhaust gas from the fuel cell system, a second passage extending from an upstream end connected to the combustion device and configured to discharge a flue gas generated in the combustion device to the outside, a joining portion in which both of the passages are joined, and a common passage extending from the joining portion to a downstream end opened to atmosphere, and configured to discharge the exhaust gas from the fuel cell system, and the flue gas generated in the combustion device to the outside; and a controller, wherein: the air intake passage and the exhaust gas passage are provided such that the air intake passage makes contact with at least a portion of the exhaust gas passage so as to allow heat exchange to occur between media flowing through the passages, and the controller is configured to cause the air intake temperature detector to detect the temperature of the intake air supplied to the case when a power generation command is input to the power generation system, and cause the combustion device to activate when the fuel cell system is activated and the temperature detected by the air intake temperature detector is equal to or lower than a first predetermined temperature. the air intake passage and the exhaust gas passage are provided such that the air intake passage makes contact with at least a portion of the exhaust gas passage so as to allow heat exchange to occur between media flowing through the passages, and the controller is configured to cause the air intake temperature detector to detect the temperature of the intake air supplied to the case when a power generation command is input to the power generation system, and cause the combustion device to activate when the fuel cell system is activated and the temperature detected by the air intake temperature detector is equal to or lower than a first predetermined temperature.