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

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

1. An opening agreement depends on the establishment of a new obligation as a dispositive act to extinguish the former obligation and to establish a new obligation. As such, when a new obligation due to an opening is not established or cancelled due to an illegal cause or a cause not known to the parties, the former obligation is not extinguished (Article 504 of the Civil Act). In the case of a so-called conditional compact with conditions attached to an opening agreement, the termination of the former obligation and the establishment of the new obligation themselves are subject to the fulfillment of such conditions (see Supreme Court Decision 2005Da31316, Nov. 15, 2007). However, a condition is an incidental member of a juristic act that depends on the nature of a new offense, and constitutes a whole content of a declaration of intent that constitutes a juristic act, and thus, the condition is required to be attached in accordance with the general principle of declaration of intent, i.e., the intent to attach the condition and its indication, even if the condition is not indicated outside, and it is merely merely an motive for a juristic act and not a condition of a juristic act.

(2) On May 13, 2003, the court below concluded an agreement between the plaintiff, stock company E (hereinafter "E") and F to jointly implement a joint housing construction project (hereinafter "project in this case") as stated in the judgment of the court below on the ground of the land of 35 lots of land (hereinafter "project site in this case"), including each land listed in the separate sheet in the judgment of the court below on January 21, 2005 (hereinafter "land in this case"), and the purchaser on January 26, 2005 as "E et al." (hereinafter "project site in this case").