An antitheft system and method capable of being more easily mounted than was conventionally done, with a detecting capability, necessary and sufficient to determine the occurrence of a theft. An RSS value r is calculated as a square-root of sum of squares by putting together each of variations in X-, Y- and Z-axes in a sensor unit 38. Then, a moving average value av obtained by averaging a predetermined number of consecutive RSS values r taken in chronological order is utilized to determine whether a theft is in process or not. Hence, even if an X-axis detector 41, a Y-axis detector 42 and a Z-axis detector 43 are not matched to longitudinal, lateral and vertical directions FR, LR and UD of a two-wheeled motor vehicle, it is possible to determine whether a theft is in process or not. Thus, the antitheft system 1 can be mounted in a desired position without taking into account a detection direction of the sensor unit 38 in relation to the two-wheeled motor vehicles 3a, 3b, thereby providing the antitheft system 1 capable of being more easily mounted than was conventionally done with a detecting capability, necessary and sufficient to determine the occurrence of a theft.