PURPOSE: To discover erroneous connection of power supply wiring quickly by connecting light emitting elements in the colours corresponding to the voltage to be connected by nature to each power supply input terminal.

CONSTITUTION: Between power supply input terminals 25 and 26 of a unit 2 a +5V power source is connected; between power input terminals 35 and 37 of a unit 3 the +5V power source is connected; between power input terminals 36 and 37 a +12V power source is connected; between power terminals 45 and 47 of a unit 4 a +12V power source is connected; between power input terminals 46 and 47 the -12V power source is connected. Light emitting elements 21 and 31 emitting yellow light are provided between 5V power connecting terminals of each unit; light emitting elements 32 and 41 emitting red light are provided between +12V power connecting terminals. A light emitting element 42 emitting green light is provided between -12V power connecting terminals. By putting each power supply to work one by one and by confirming it is only light emitting elements corresponding to the colour that emit light erroneous wiring of power distribution is checked.