The step-up method is used to estimate the impulse breakdown voltages when the electrical insulation is not usable after it is broken. This paper analyses the reliability of the estimates of the underlying breakdown probability distribution in the step-up method, when (1) the observed breakdown voltage itself is available and (2) it is not available. The former case has many advantages compared to the latter case such that (1) the confidence intervals of the estimates become smaller and (2) the estimates can be obtained in stable. This paper summarizes the results of the three cases that the underlying probability distribution for the breakdown voltage is assumed to be normal, Weibull, and gumbel types. The optimal test method is simply and clearly described for various distribution models.