Abstract: This key security device is used together with a first terminal which may be a mobile phone with high functionality and a second terminal which may be an electronic key. The key security device is provided with: a location information acquiring unit which acquires location information about the first terminal from a global positioning system (GPS) receiver of the first terminal; a level setting unit which sets an alert level according to the acquired location information; and a security guard processor which sets a location determination parameter value according to the alert level, monitors the distance between the first terminal and the second terminal through communication between the first terminal and the second terminal according to the alert level and the location determination parameter, and executes a security guard processing of the second terminal according to the result of the monitoring.