PURPOSE: To reduce the occurrence of errors generated depending upon the dynamic range of a voltage measuring instrument by adding an offset voltage and signal voltage to a DUT and eliminating the offset voltage from the output voltage of the DUT.

CONSTITUTION: An L-power source 2A generates an L-voltage and an H-power source 2B generates an H-voltage. An offset power source 2C generates an offset voltage. Offset adder circuits 3A and 3B respectively add the offset voltage to the L- and H-voltages. Offset elimination circuits 4A and 4B add the outputs of the circuits 3A and 3B to a DUT 10 by using patterns from a pattern generator 1B and subtract the offset voltage from the output of the DUT 10. Voltage measuring instruments 5A and 5B measure the outputs of the circuits 4A and 4B.