To provide an antitheft system, a reader and an antitheft method for determining the abnormal condition or not of an object and facilitating the antitheft operation of preventing the object from being stolen and searching the stolen object by comparing object information with antitheft information.

The antitheft system is mounted on the object for storing the object information, In antitheft operation using an antitheft tag for communicating the object information with the outside by means of electromagnetic waves or an induction field, the object information is read out of the antitheft tag, the read object information is compared with the antitheft information for determining the abnormal condition or not and the determination result is output.