Abstract: A security test can be performed on an ECU by using random test signals without changing the ECU. A security test system for a control device for an on-vehicle network, includes a security test unit that includes a test signal transmission portion transmitting random test signals including an illegal signal, for diagnosing security of a control device for an on-vehicle network, and a diagnosis portion specifying the illegal signal on the basis of a process result in the control device using the test signals, and a function evaluation unit that includes a process information acquisition portion acquiring information regarding the process result in the control device, an abnormality detection portion detecting abnormality in the process result in the control device based on the test signal, and an abnormality information output portion outputting information regarding the detected abnormality.