This article relates to the mutual relation of a registered trademark owner and a prior user who has established his own reputation through using trademark in good faith. As to Trademark Act, the owner of a trademark right is entitled to the exclusive right to use the registered trademark for the designated goods. Therefore, under Trademark Act, only registered trademarks are directly protected. However, trademarks that are identical or similar to another person`s trademark when that other person`s trademark is well known among consumers to indicate or resemble the other person`s goods, and when the trademarks are to be used on goods that are identical or similar to such goods, trademark registration may not be obtained and consequently well known trademarks are also protected under Trademark Act indirectly. Meanwhile, an act of causing confusion with another person`s goods by using signs identical or similar to another person`s name, trade name, trademark, container or package of goods or any other sign widely known as an indication of goods, or by selling, distributing, importing or exporting goods with such signs is prohibited by Unfair Competition Prevention and Trade Secret Protection Act. Under these circumstances, if there are a prior user and a registered trademark owner as to identical or similar trademark, conflict of the above two laws is unavoidable. This article suggests reasonable interpretation of Trademark Act and Unfair Competition Prevention and Trade Secret Protection Act to balance the right of registered trademark and trademark which is well known but unregistered, resulted in preventing unfair competition and contributing to the development of industry.