A security system and a method of determining whether there is an intrusion are provided. A correlation between sound signals obtained from a sound source generating device and a sound-field variation measuring device is used to measure a acoustic field in a certain space. A difference between an initially set acoustic field and a changed acoustic field is used to determine whether there is an intruding object in a certain space. The security system includes a sound source generating device for generating a sound source; and a sound-field variation measuring device for measuring a acoustic field formed by the sound source, wherein an initially set acoustic field is compared with the measured acoustic field to determine whether there is an intrusion.