When the owner of real estate makes a sales contract with a purchaser, he should complete the registration of transfer of ownership for the purchaser according to the condition of the contract. This means that the seller must not fail to perform his juristic duty for the purchaser. If the seller of real estate who once has made a contract with one purchaser makes the same contract with another purchaser and completes the registration of transfer of ownership for the second purchaser, as occasion calls, the latter contract with the second purchaser is null and void. This is the kind of case which is just related with the problem of the double sale of real estate. In korea, almost all judicial precedents have declared that the second contract in the double sale of real estate may become void in order to protect the first purchaser, if circumstances require. For instance, when the second purchaser incites the owner not to carry out the sales contract of real estate with the first purchaser or participates in the owner's breach of contract with the first purchaser actively, thus, from the owner, he buys the same real estate for himself with a fraudulent measure, the second purchaser's contract, that is, the latter contract is null. What is more, the nullity of contract is absolute in it's nature, so, according to this principle, even the innocent third person who is connected with the continuous transaction of the real estate afterward, can not oppose the first purchaser. This theory is called the principle of nullity of juristic act against social order. And the theory of judicial precedents is based on Article 103 of the Civil Law, thus, by the theory, sometimes the double sale of real estate may be juristic act against social order. Since early time, many scholars have almost the same views as judicial precedents. But now a day, it was revealed there were many problems in the common views and judicial precedents. First of all, the principle interferes the transaction safety of real estate seriously. Therefore some scholars assert different views from common views about the double sale of real estate to protect the innocent third person, but it is not enough to solve the problems mentioned above. This study discourses upon the basic solution of the problems to keep the safe of transaction of the real estate. In conclusion, I'd like to say the following things; always the double sale of real estate is not juristic act against social order, and is not under Article 103 of the Civil Law, the double sale of real estate is not occasionally invalid but always valid, therefore, now such a doctrine as the judicial precedents ought to be abolished.