A plurality of terminal devices 3 are connected to a parent device 1 to measure analog voltages of the terminals. Reference voltages 6 and measurement voltages 7 each having a polarity reverse to that of clocks are sequentially switched to be connected at gaps of clocks supplied by the parent device 1. Thus, the reference voltages 6 and the measurement voltages 7 of the terminal devices 3 are directly connected. With this mechanism, measurement of analog measurement signals at multiple points can be performed by use of a pair of electric wires, in which errors due to voltage drops of the measurement voltages 7 over the transmission line 2 are corrected with measurements of the reference voltages 6 whose voltage is known in advance so that precise analog measurements can be performed.