Executive stock option was introduced in Korean corporate law in 1997 to provide directors and officers with proper incentive to maximize firm value. General rules said that the option holders who are eligible to execute the options should maintain their position for 2 years or more since their reception, but special rules applicable only to listed companies allow the option holders who retire within this 2 years to execute the options if such event is not according to their own decision (involuntary retirement). Recently, the Korean Supreme Court held that such special treatment shall not be applied to non-listed companies, since the general rules do not mention about involuntary retirement. This paper identifies two legal issues in this decision: (1) what the proper interpretation of the general rules is if the parties does not negotiate on this issue, and (2) whether the parties are prohibited from negotiating and reaching conclusions that are contrary to the general rules. The second issue is about the mandatory nature of corporate law. This paper argues that the court decided correctly, if the parties did not negotiate, because the special rule explicitly inserts the words, “notwithstanding the general rules”, which implies that the general rules should be understood differently. If, however, the parties negotiate and the articles of incorporation or general shareholders' meeting provides that the option holders are eligible to execute the options in the event of involuntary retirement, this agreement should be valid. In fact, the rules on stock option is never the core provisions of corporate law, and in this decision the court may be understood to declare that most corporate law provisions are mandatory by holding that even such negligible provision is mandatory rule.