An ECU monitoring device includes a monitoring ECU connected to a power line for supplying power from a main battery to an immobilizer ECU. The monitoring ECU includes a high-frequency signal generating circuit for superimposing a high-frequency signal on the power line, a high-frequency signal detecting circuit for detecting the high-frequency signal superimposed on the power line and a CPU for judging a connection between the power line and the immobilizer ECU or between the power line and a false instrument. With the constitution, the ECU monitoring device can detect a situation that the immobilizer ECU is detached from a vehicle with high accuracy, preventing the vehicle from being stolen by theft.