To easily obtain a quantity of received light with computation by only measuring pulses of an electric signal related to a flame sensor, a flame detecting system is disclosed comprising: a flame sensor to detect light and a calculating device, in which the calculating device includes an applied voltage generating portion configured to generate a pulse to drive the flame sensor, a voltage detecting portion configured to measure an electric signal flowing in the flame sensor, a storing portion configured to store sensitivity parameters of the flame sensor in advance, and a central processing unit configured to obtain a quantity of received light of a flame using parameters of a known quantity of received light, a pulse width, and a discharge probability of the sensitivity parameters, and a discharge probability obtained from an actual pulse width and the measured number of discharge times.