Disclosed is a human sensing back-up alarm system capable of generating a loud alarm sound for a predetermined period of time using a speaker to protect humans or vehicles if the humans or vehicles are detected behind construction equipment or a vehicle when the construction equipment or the vehicle backs up, and generating a low alarm sound if the humans or vehicles are not detected behind the construction equipment or the vehicle, thereby effectively preventing accidents while minimizing ambient noise and noise pollution. The human sensing back-up alarm device includes a PCB control part for controlling an alarm sound, a human sensing part for electrically sensing an optical signal, and a speaker part for converting an electric signal into a mechanical signal. The PCB control part includes a clock generator for generating the alarm sound, a clock divider for dividing clocks generated from the clock generator, a terminal unit, a loudness control unit for adjusting the loudness of the alarm sound, a non-continuous sound control unit for generating a non-continuous sound, a speaker drive unit for driving the speaker part and a power source for stabilizing power applied to a vehicle.