There are three factors required to be trade secret. namely, a trade secret is information that (1) is not generally known to the public (2) confers some sort of economic benefit on its holder (3) is the subject of reasonable efforts to maintain its secrecy. If an employee uses the secret information of his company by a dishonest means or he violates his obligation to keep trade secret, he trespasses trade secret of his company. This article focuses on court`s decision pattern on definition and infringement of trade secret. what is more, It deals with Japanese court`s decision comparing with Korean court`s decision.