PURPOSE: To achieve tests of an saturated large current and non-saturated voltage by interrupting a power source input in a short time less than the thermal time constant.

CONSTITUTION: A power source 7 of the max. voltage is connected to a power source terminal of a semi-conductor device 5 to be tested and a load impedance 6 through which the max. current flows when an input source 8 is connected to an input terminal to saturate the device 5 amply is connected to an input terminal. Then, when the power source 8 is interrupted for a short time less than the time constant of the device 5, tests for saturated large current and non- saturated high voltage of the device 5 can be performed alternately without temperature changes due to heat generation.