PROBLEM TO BE SOLVED : To provide an illegality detection system and an illegality detection method capable of, when the address of the delivery destination of merchandise or the like is illegally used, detecting the illegality.SOLUTION : An illegality detection device 1 includes : an advertisement carriage information database for storing advertisement carriage information including the advertisement carriage period of each real estate property; address acquisition means for acquiring an address designated as the delivery destination of merchandise by the applicant of transaction; use period acquisition means for acquiring the use period of the address by the applicant; advertisement carriage determination means for determining whether or not real estate property related to the address acquired by the address acquisition means is carried in advertisements on the basis of the advertisement carriage information stored in the advertisement carriage information database; and illegality detection means for, when it is determined that the advertisements have been carried, determining whether or not the address has been illegally used on the basis of the advertisement carriage period and the use period of the address acquired by the use period acquisition means.COPYRIGHT : (C)2015, JPO&INPIT