To remove an influence of an offset value of a voltmeter in a measurement of an output voltage of an element to be inspected.

First of all, a relay RLY11 is closed, a relay RLY12 is opened, and a voltage Vt1 between a reference potential terminal T1 and a predetermined voltage terminal T3 is measured by a voltmeter 2. Then, the relay RLY12 is closed, the relay RLY11 is opened, and a voltage Vt2 between the predetermined voltage terminal T3 and a measuring object voltage terminal T2 is measured by the voltmeter 2. An arithmetic processing unit 43 calculates a difference between the measured values of the volts Vt1 and Vt2 to obtain a voltage V between the reference potential terminal T1 and the measuring object volt terminal T2 and also offset an offset voltage contained in the respective measured values.