To easily detect erroneous assembly of a plurality of electronic components, which are used as a set, without inputting a specific ID code.

An MPU 13 of a slave unit 5 and an MPU 23 of a master unit 7 respectively calculate a power supply time from when an ignition switch 3 turns on to when the ignition switch 3 turns off and store in EEPROMs 15, 25. The slave unit 5 transmits the power supply time stored in the EEPROM 15 to the master unit 7 by a communication circuit 17, and the master unit 7 receives through a communication circuit 27. The MPU 23 of the master unit 7 compares the power supply time received from the slave unit 5 and the power supply time of the master unit 7 stored in the EEPROM 25. When there is a great difference, it is determined that either of the slave unit 5 or the master unit 7 is assembled erroneously and an alarm is outputted from an alarm device 33.