In-Vehicle System and Electronic Control Device

When occurrence of a security event related to a security abnormality is detected, a detection device transmits a countermeasure request message including detected content of the security event to a countermeasure reader device. When the countermeasure request message is received from the detection device, the countermeasure reader device specifies countermeasure content for the security event and transmits a pre-preparation message including the detected content and the countermeasure content to other countermeasure devices. When each of the countermeasure devices excluding the countermeasure reader device receives the pre-preparation message from the countermeasure reader device, the countermeasure device verifies consistency between the detected content included in the received pre-preparation message and the countermeasure content.