A flame detection system includes: a UV sensor that serves as a flame sensor detecting a UV ray generated by a flame; an application voltage generation unit that applies a driving voltage to the UV sensor; a discharge detection unit that detects a discharge in the UV sensor; a discharge count unit that counts the number of detected discharges; a discharge probability calculation unit that calculates a discharge probability on the basis of the number of discharges counted by the discharge count unit and the number of times the driving voltage is applied; a UV intensity determination unit that determines an intensity of the UV ray on the basis of the discharge probability; and a determination result output unit that outputs the intensity of the UV ray determined by the UV intensity determination unit via display or communication.