PURPOSE: To save a power loss caused by the conversion of the level of voltage by providing DC output voltage from each DC-DC converter for at least a part of the electrical device of a corresponding terminal device.

CONSTITUTION: The DC-DC converter 19 is provided with the high voltage of 48V from a power supply line E and has the well-known constitution of converting the high voltage of 48V into the DC low voltage VL of 10V necessary for making each circuit of a detection terminal device T1 to work. The DC low voltage VL is provided for each part of the detection terminal device T1 to enable the detection terminal device T1 to execute prescribed operation. As the DC-DC converter 19 converts provided high voltage into required DC low voltage VL, conversion loss can be small as compared with the case of using constant voltage power which consumes an excessive voltage as heat in a resistor.