Abstract: A clone terminal detection system includes a position information memory, comparator, and detector. The position information memory updates and stores position registration information of a mobile terminal, which includes a telephone number, on the basis of a position registration request from one of the mobile terminal and a clone terminal illegally copied from the mobile terminal. The comparator compares position registration information of an origination request terminal with the position registration information stored in the position information memory at the time of origination request from one of the mobile and clone terminals. The detector detects the presence of the clone terminal having the same telephone number as that of the mobile terminal on the basis of the comparison result from the comparator. A clone terminal detection method is also disclosed.