A comparison unit (22) compares a measurement value, which is one of changes over time in a voltage value of a bus line (11), distance transition of the voltage value, changes over time in an impedance value of the bus line (11), and distance transition of the impedance value that have been measured, with a normal value, which is one of changes over time in a voltage value of the bus line (11), distance transition of the voltage value, changes over time in an impedance value of the bus line (11), and distance transition of the impedance value in a normal state, and extracts a difference between the measurement value and the normal value. An unauthorized connection determination unit (24) evaluates the difference between the measurement value and the normal value extracted by the comparison unit (22), so as to determine whether an unauthorized device is connected to the bus line (11).