A light signal detecting circuit, a light amount detecting device, and a charged-particle-beam device capable of discriminating the signal component of a small amount of light from the signal component of noise due to dark current. A data-processing-unit detects pulses from digital voltage signal corresponding to an amount of light obtained by an amplifier and an A-D converter, calculates a crest value as the maximum voltage value of each pulse, and stores the occurrence frequency of each calculated crest value in a frequency occurrence storage area. A data analysis unit compares a previously-determined frequency lower limit with the occurrence frequency of each crest value in ascending order of the crest values and sets a pulse determination threshold to the first crest value whose occurrence frequency is equal to or smaller than the frequency lower limit. The threshold processing unit thus outputs the digital signal higher than the pulse determination threshold.