The fetal implant of soul is accepted as the natural process of soul reincanation to some religions, whereas 'walk-in' is considered an erratic phenomenon. Although records of such 'walk-in' appear sporadically in ancient Chinese literature, considering the natural cycle and replacement of non-meterial souls and materal bodies, it seems there is an inevitable principle that has been applied to them. That is, if a soul has to go through a certain amount of karma in its life, a physical time is allotted so that one can finish it. As we put it together, if we recall the first question we had about this phenomenon, why doesn't Spiritual World lead him to a new fetus and live a new life, even if it's a spirit that dies without a fixed life span? Suppose people die unexpectedly before fully living their given life span, regarding the principle mentioned we can only wonder, why doesn't the Spiritual World lead the souls to a new fetus and let them live a new life? The question is why souls are allowed to put into a dying body under the condition similar to a previous life. As a result of a sumarizing and analyzing the above various cases, my view is that if souls plan to live a certain physical time on a planet, the mission to complete the physical life process designated for those souls will remain in that planet's time. Otherwise, there is no reason for the administrators of the Spiritual World to give the souls more time for physical life in a way called 'walk-in'. In the Spiritual World, the 'walk-in' phenomenon did not just give time for recycling body and physical life. As seen in the case of Jukgyejeong, he hears from the administrators of the Spiritual World that the purpose of 'walk-in' is to let people “know what is good and what is sinful” by reincarnating through other people's bodies.. The same sayings are recorded in the case of bang's daughter. Lee's daughter-in-law's spirit is put into Bang's daughter on the grounds that “it would be difficult to encourage people to live doing good deeds” unless she is revived through other's body and succeed the generation. In many other records, the main point of the conversation by dead spirits with the managers of Spiritual World after their arrival to the Spiritual World was the inspection of good and evil deeds. In this regard, there is a consistent standard in the Spiritual World, that is poetic justice. In general, there is a view that this record in ancient literature is made up to teach “know what is good and what is sinful”, however, in actuality, the principle of the Spiritual World itself was “know what is good and what is sinful”, and the basic laws of all life phenomena encouraged good and punished evil.In other words, to summarize the above studies, it can be seen that 'walk-in' gives a few dead people an apportunity for spiritual growth as a result of good deeds by writing off Karma for the rest of their lives even with the body of others.