A security device includes a plurality of security detectors intercommunicating with each other. Each of the security detectors includes a first device for verifying a single zone verification time of the respective security detector and a second device for verifying a multiple zone verification time with another security detector corresponding to a distance between two security detectors at two different detecting areas. When one of the security detectors detects at least two triggered signals in the respective detecting area within the single zone verification time, the respective security detector activates the local warning system to produce a local warning signal. When two security detectors are intercommunicated with each other to detect two triggered signals in the detecting areas respectively within the multiple zone verification time, at least one of the security detectors activates the local warning system to produce the local warning signal.