A flue gas detection system includes: a temperature sensor configured to detect a temperature in a duct; a temperature sensor configured to detect a temperature in a duct; and an ECU. The ECU is configured to determine that a high-temperature gas is released from an assembled battery, when a period during which a temperature increase amount in the duct is more than a reference amount and a period during which a temperature increase amount in the duct is more than a reference amount are included in a predetermined period.