1. The defendant's appeal is dismissed.

2. The costs of appeal shall be borne by the Defendant.

3. The text of the judgment of the first instance.

1. The reasons for the court's explanation concerning this case are as follows. The defendant's argument in the trial of the court of the first instance is as stated in Article 420 of the Civil Procedure Act, except for adding the following judgments as to the defendant's argument in the trial of the court of the first instance, it shall be accepted in accordance with the main sentence of Article 420 of the Civil Procedure Act.

2. Additional matters to be determined;

A. As to the assertion of gross negligence, the Defendant asserts that the instant contract cannot be revoked on the ground of mistake, since the Plaintiff had already been aware of the ownership relationship, status, and road plans of the instant land and buildings as residents of the area where the instant land and buildings are located, and that the instant contract was concluded by gross negligence, even though he could have easily known information about the area of the instant building that could have been newly constructed in the instant land if he had been asked for viewing.

The term "serious negligence" as stipulated in the proviso of Article 109 (1) of the Civil Act refers to the failure of attention that is ordinarily required in light of the name of the voter, the type, purpose, etc. of the act (see, e.g., Supreme Court Decision 96Da26657, Aug. 22, 1997). The following circumstances, i.e., the general person who, among real estate, requests a brokerage to a private individual, conducts a sale and purchase contract by requesting a brokerage to a private individual, is believed to believe that the real estate broker is trusted to have professional knowledge and experience, and that there is no error in the instruction and explanation of the terms of the transaction by his involvement. ② The plaintiff requested a licensed real estate agent I to act as the broker of the contract of this case. ② The plaintiff is the representative of G, who is the defendant with the above I and licensed real estate agents, and the size of the F land covered by the building of this case is limited to 3.7 square meters, and the remaining building site of this case and this case except the building site of this case.