The conventional step-up test method has been used to estimate the impulse breakdown voltages used in the field of the field of electrical insulation engineering. This method uses two-valued discrete data according to the result of insulation broken or non-broken information. We can now accurately measure impulse waveforms by recently developed fast memorable measuring equipment. With this equipment, we can observe the continuous breakdown voltage data, resulting in more accurate breakdown voltage estimates. We call this method the new step-up test method. This article discusses the optimum test method in both the conventional and new test methods, and compare the estimating errors of the low probability breakdown voltages in both methods.