A control substrate is provided with a first electric current limiting resistor and a first voltage limiting Zener diode which are connected to a power supply line, and second electric current limiting resistors and second voltage limiting Zener diodes which are connected to communication lines. Of an electronic circuit configured from the control substrate and a sub-substrate, a first area connected to one side of the power supply line and the communication lines and a second area connected to the other side thereof with reference to the first and second electric current limiting resistors R11 to R13 and the first and second voltage limiting Zener diodes are arranged spaced apart from each other by a predetermined interval so that the areas are considered separate areas in terms of intrinsically safe explosion-proof requirements.