PURPOSE: To enable automatic and continuous recording of a dielectric breakdown voltage in a digital value by providing a transformer, an automatic voltage booster, a current transformer, a digital voltage display, a memory device, a plotter and a printer.

CONSTITUTION: A voltage rising speed of an automatic voltage booster 2 is set and then, a primary side voltage of a transformer 1 is risen with a starting button not illustrated. The rising value of the secondary side voltage of the transformer 1 is indicated continually on a digital voltage display 6. At the same time, the voltage value is memorized 7 at a sampling interval of a fixed time. When a dielectric breakdown occurs in a smaple 4a at a sample mounting section 4, a current flows through a secondary side circuit of the transformer 1 and detected with a current transformer 5. The detection signal is transmitted to the device 2 and the device 7 and the device 2 is stopped while the memory action of the device 7 is halted. At the same time, a signal of a dielectric breakdown voltage value stored in the device 7 is transmitted to a plotter 8 and a printer 9. The plotter 8 make the dielectric breakdown voltage value into a graphical from while the printer 9 prints out the results as digital value.