The judgment below is reversed and the case is remanded to Seoul High Court.

The grounds of appeal are examined (to the extent of supplement in case of supplemental appellate briefs not timely filed).

1. Where a contractual party prepares in writing a certain content of a contract as a disposal document, if the objective meaning of the text is clear, barring any special circumstance, the existence and content of the declaration of intent shall be recognized pursuant to the said text. However, if the meaning of the text is not clearly expressed, it shall be reasonably interpreted in accordance with logical and empirical rules, and social common sense and transaction norms, by comprehensively examining the contents of the text, motive and background of the contract, the purpose and genuine intent that the contractual party intended to achieve by concluding the contract, transaction practices, etc.

In particular, if the content of a contract claimed by one party imposes a serious liability on the other party, the content of the contract should be interpreted more strictly.

(2) In order to cancel a contract on the ground of nonperformance pursuant to Article 544 of the Civil Act, the contract shall not be rescinded unless the performance of the obligation in question is performed, and it shall be the principal obligation to the extent that the obligee would not have concluded the contract. The contract shall not be rescinded unless the performance of the obligation in question is performed, and it is merely for the non-performance of the incidental obligation.

In addition, in distinguishing whether a contractual obligation is a principal obligation or an incidental obligation, regardless of the independent value of the benefit, it shall be determined by the reasonable will of the parties expressed or objectively expressed in the situation at the time of the conclusion of the contract, and such various circumstances as the content, purpose, and the result of the nonperformance shall be considered

(see, e.g., Supreme Court Decision 2005Da53705, 53712, Nov. 25, 2005). 2. The lower court determined as follows.