1. A security monitoring system using beamforming acoustic imaging, the system comprising: a primary detection device that initially detects a trespasser in a space; an acoustic generating device that generates acoustic signals in response to the initial detection of the trespasser in the space, wherein the acoustic generating device generates the acoustic signals periodically for monitoring a position of the trespasser after initial detection of the trespasser in the space; a plurality of acoustic measuring devices that receive the acoustic signals; and an acoustic image processing device that periodically generates acoustic images that include a distribution of the acoustic signals by applying a beamforming algorithm to the acoustic signals received by the plurality of acoustic measuring devices and determines the position of the trespasser in a space by comparing a reference acoustic image of the space generated before the trespasser is detected and a generated acoustic image of the space generated after the trespasser is detected, wherein the acoustic image processing device includes a fixed beamformer or an adaptive beamformer detecting the acoustic signals in the plurality of acoustic measuring devices, dividing the detected acoustic signals into a specific frequency component using fast fourier transform and performing angle division on a specific angle direction, thereby obtaining the distribution of the acoustic signals.