A fraud detection network system comprising a web server device configured to transmit a question that is randomly selected from combinations of a plurality of questions previously allocated for each user and correct answers therefore, a user terminal device connected with a network, the user terminal configured to return the correct answer such that authentication processing is performed, the system being configured to thereafter provide a service and comprising a fraud detection server connected to the network and configured to detect an unauthorized operation in the user terminal device, wherein the web server device includes: a storage means for storing web content information used for the provision of the service and a call script for calling the fraud detection server device; and a content transmission means for transmitting, after secure communication with the user terminal device is established, to the user terminal device, the web content information including the call script, the user terminal device includes: a content reception means for receiving, after secure communication with the web server device is established, the web content information including the call script; a web browser processing means for displaying the web content, for receiving an information input from the user and for performing the call script; and a fraud detection server communication means for communicating with the fraud detection server device, the user terminal device is further configured to, based on the call script, acquire a fraud detection script from the fraud detection server, based on the fraud detection script, search detection target data included in the web content information, transmit a result of the search to the fraud detection server device and perform a predetermined fraud measure operation based on a fraud detection result received from the fraud detection server device, and the fraud detection server device includes: a fraud detection script storage means for storing the fraud detection script corresponding to the call script; a user terminal communication means for communicating with the user terminal device; and a fraud detection means for detecting, on the result of the search by the fraud detection script, whether or not the unauthorized operation is present. a storage means for storing web content information used for the provision of the service and a call script for calling the fraud detection server device; and a content transmission means for transmitting, after secure communication with the user terminal device is established, to the user terminal device, the web content information including the call script, the user terminal device includes: a content reception means for receiving, after secure communication with the web server device is established, the web content information including the call script; a web browser processing means for displaying the web content, for receiving an information input from the user and for performing the call script; and a fraud detection server communication means for communicating with the fraud detection server device, a content reception means for receiving, after secure communication with the web server device is established, the web content information including the call script; a web browser processing means for displaying the web content, for receiving an information input from the user and for performing the call script; and a fraud detection server communication means for communicating with the fraud detection server device, the user terminal device is further configured to, based on the call script, acquire a fraud detection script from the fraud detection server, based on the fraud detection script, search detection target data included in the web content information, transmit a result of the search to the fraud detection server device and perform a predetermined fraud measure operation based on a fraud detection result received from the fraud detection server device, and the fraud detection server device includes: a fraud detection script storage means for storing the fraud detection script corresponding to the call script; a user terminal communication means for communicating with the user terminal device; and a fraud detection means for detecting, on the result of the search by the fraud detection script, whether or not the unauthorized operation is present. a fraud detection script storage means for storing the fraud detection script corresponding to the call script; a user terminal communication means for communicating with the user terminal device; and a fraud detection means for detecting, on the result of the search by the fraud detection script, whether or not the unauthorized operation is present.