A security system includes an information transmission device, and a security device to which a target device is connectable. The information transmission device and the security device are connected to each other via a network. The information transmission device transmits, to the security device, generation information to be used for generating reference identification information. The security device generates the reference identification information by using the generation information received from the information transmission device via the network. The security device acquires identification information of the target device connected to the security device. The security device compares the acquired identification information with the generated reference identification information, and determines validity of the target device connected thereto, based on a result of comparison.