ECU, MONITORING ECU, AND CAN SYSTEM

An object of the present invention is to accumulate data in an ECU related to a CAN message transmitted by an ECU in a CAN system. An ECU according to the present invention is an ECU among a plurality of ECUs connected to a CAN communication line, the ECU includes a transmission unit configured to transmit first CAN messages to other ECUs of the plurality of ECUs, and an accumulation unit configured to accumulate CANIDs of the plurality of first CAN messages transmitted by the transmission unit as transmission CANIDs.