Abstract: An intrusion detecting method using an intrusion detecting system which includes an optical fiber line installed in an area in which intrusion is to be detected, a laser diode inputting an optical signal to the optical fiber line, a photodiode detecting a reflected optical signal when the optical signal is reflected from the optical fiber line, an intrusion determining unit comparing a waveform of the reflected optical signal detected by the photodiode with a set reference waveform to determine whether an intrusion has occurred, and a laser diode temperature measuring unit measuring a temperature of the laser diode, includes designating, by the intrusion determining unit, a temperature of the laser diode at a first point in time as a first temperature and setting, to a reference waveform, a waveform of a first reflected optical signal caused by an input optical signal input by the laser diode at the first point in time, designating, by the intrusion determining unit, a temperature of the laser diode at a seco