In connection with the policy of the Ministry of Justice that has promoted parole, Parole has been discussed actively since 1984. However, there are very few researches that indicate positive fundamental data supporting the change of the policy.

So far, the positive analyses related with the parole were apt to use independent indices, such as the rate of parole, executed rate of parole and recidivism rate of parolees. Therefore a point of view unifying various indices seemed to be defective.

In this study, we connected various indices with a single model and analyzed the interaction of them by applying a signal detectability model (SDT) to the application of parole. Furthermore, we assessed the secondary effect of the active use of parole.

It was revealed that the increase of recidivism rate of parolees was small in comparison with the increase of rate of parole , and that active use of parole produced certain effect on the reduction of prison population.