1. A security system comprising: a first information collecting module configured to cyclically collect, as target-object information, information indicative of environments of a target object and a monitored area in which the target object exits; an alarm module configured to: deter mine whether there is an abnormal situation associated with at least one of the target object and the monitored area based on the target-object information collected by the first information collecting module, and set off an alarm upon determination that there is an abnormal situation associated with at least one of the target object and the monitored area; a second information collecting module configured to cyclically collect, as circumstance information, information indicative of a situation of an area surrounding the monitored area; and an abnormal situation determining module configured to: calculate, based on the circumstance information collected by the second information collecting module, a probability that an abnormal situation associated with at least one of the target object and the monitored area will occur; and determine whether the probability that an abnormal situation associated with at least one of the target object and the monitored area will occur is equal to or higher than a threshold level.