This paper is to discuss essential business of hospital business. While the labor world and ILO made continuous recommendation for improvements towards the compulsory arbitration system along with the controversy over unconstitutionality of the system, the Constitutional Court ruled that the system is constitutional on December 23, 1996(90hunba19) and on May 15, 2003 (2001hunga31). Despite this decision from the Constitutional Court, there has been much controversy over whether the compulsory arbitration system infringes the rights of collective action against the principle of trade union & labor relations adjustment which allows Commissioner of the Labor Relations Commission to decide on submission of arbitration by virtue of his/her authority in case where industrial disputes take place in the essential public-service businesses. The revision on the above provision was closely examined from the year 2003 and an agreement was made on the abolition of the compulsory arbitration system and the introduction of essential business with a grand compromise among labor unions, employers and the government on September 11, 2006 followed by revision(Essential business system enacted on January 1, 2008) of the Trade Union & Labor Relations Adjustment Act on December 30 in the same year. Accordingly, in order to perform the essential business, parties to labor relations must have an agreement or obtain a decision by the Labor Relations Commission before taking industrial actions. This paper firstly examined the concept of essential public-service businesses and essential business, legal meaning of essential business, procedures for making agreement and decision and legal effects. Secondly it intensively explored a theory against the principle of the legality which was raised from some part of society. In other words, it is claimed that a theory against the principle of the legality is not consistent with the rule of legislation and some abstract wording is against void for vagueness doctrine because part of crime constitution requirements is delegated to the Presidential Decree or to consultation among parties to labor relations. But analysis on the rule of legislation and void for vagueness doctrine reflected in the decision by the Constitutional Court led that argument for a theory against the principle of the legality is not reasonable. Close examination was done on a formal act of essential business agreement and necessity of prior agreement before submission of decision to the Labor Relations Commission which might have difficulties in performing work. In addition, an example agreement on hospital essential business is attached to help you understand this paper better.