1. A security system using measurement of acoustic field variation, the security system comprising: a sound source configured to generate an acoustic field; and a plurality of sound-field variation measuring devices configured to measure an acoustic field formed by the sound source inside a certain space including at least four corners, and to compare the measured acoustic field with an initially set acoustic field to determine whether there is an intrusion in the certain space, wherein the plurality of sound-field variation measuring devices include at least four sound-field variation measuring devices respectively disposed in the four corners of the certain space, and wherein the sound source is configured to generate a pink noise or a white noise in a certain frequency band.