PURPOSE: To reduce the required number of switch elements corresponding to a high voltage by selecting both of a high voltage system and a low voltage system at the first switch element and selecting only the low voltage system at the second switch element.

CONSTITUTION: In the case of selecting a 1000V range, the movable contact of a relay RL 1' is connected to the ON side. Thus, a feedback resistor R1 of 10MΩ is connected to a feedback loop on the side of an amplification part U1, and the amplification factor becomes 100. In the case of selecting a 100V-1V range, the movable contact of the relay RL1' is connected to the OFF side. Thus, the low voltage system is selected. Then, relays RL2'-RLn' are successively turned on corresponding to the desired range. Thus, the switch element to be impressed at a high voltage 1000V is only the relay RL1' corresponding to the high voltage to be used as the first switch element, and a low voltage (maximum 100V) is impressed to the relays RL2'-RLn' to be used as the second switch elements.